The several reasons which mention spirituality certainly speak to me.
I copied the whole list, but the formatting didn't come, so go look at the link to see a more beautiful version.
People run ultras for many different reasons, and we are rarely aware of our own motivation. The answers below reflect some of the possible rationales behind this grueling sport. The runners I know are driven by various combinations of these forces, and the specific mix is unique to each individual.
Health. You might expect people to run ultras for their health, but most ultrarunners do far more than is required for optimum health. While it's common for ultrarunners to have started running for their health, and for health to be a strong motivation to keep running, it's not the reason why we move to running the extreme distances of ultramarathons. In fact, for many of us, running is the motivation to keep healthy rather than the other way around.
Fitness. Ultrarunning takes fitness far beyond what is required for anything other than running ultras. Also, while Ultrarunning requires remarkable levels of endurance and stamina, it's not a well-rounded fitness, and many ultrarunners lack other components of fitness such as upper body strength.
The Runner's High. The runner's high is real, but it's a rarity and normally associated more with short, fast runs than the Long Slow Distance that most Ultrarunning involves. In fact, most Ultramarathon races involve depression and fantasies of self-harm. ("If only my leg would break, I could stop…" type of thoughts.)
The Difficulty. When JFK announced the mission to the moon, he said "we choose to do these things, not because they are easy, but because they are hard." One of the things that drive many high performing individuals is the need to overcome and achieve things that are difficult, and Ultrarunning clearly fulfills that need. This also reflects the desire to find progressively tougher races and longer distances.
The Monastery. The training required for Ultrarunning can provide a monastic like structure to life. Everything becomes ordered around training, with food and drink becoming more a part of the training regime than for pleasure or sustenance. This regimen removes the burden of daily decisions and creates a harmonious routine.
The Voices. There are well documented mental health benefits to running, and there are anecdotal comments that suggest many people use Ultrarunning to "keep the voices quiet." It's unclear if there are real problems with schizophrenia, or just the desire for mental peace that drives many to run vast distances.
Antidepressant. Because of the mental health benefits of running, it can be used to help treat depression. It's been suggested that ultrarunning is especially appealing to those with depression that manifests itself in self-destructive forms.
The Affliction. While running can help with a number of mental health problems, it can sometimes be a manifestation of the illness. This is especially true for eating disorders, such as anorexia or bulimia. The line between ultrarunning as treatment or affliction is sometimes unclear, and I would highly recommend leading Pam Reed's book The Extra Mile.
Movement. There is a simple pleasure in movement, and some people find it difficult to be stationary for protracted periods of time. Sometimes it seems like we have to be moving to be still, an idea encapsulated in the Zen principle of Stillness in Motion.
Nietzsche. The famous quote "what does not kill us makes us stronger" is particularly appropriate for Ultrarunning. However, I'd argue that it's not the physical strength that Ultrarunning increases, but the mental and spiritual strength. When you reach the point of such physical exhaustion and pain that you weep out loud and start to wish for death but somehow carry on, it profoundly changes who you are. I've seen the physically strong give up when they had resources left, having simply lost the will to keep moving. They measured themselves against the distance and came up wanting. But I've seen others who had been torn apart both physically and mentally by the distance who simply refused to quit, and they were victorious. The acts of determination and courage I've seen in ultramarathons are both humbling and inspiring.
The community. Ultrarunning is the most supportive and collaborate individual sport I know of. With the exception of the few who hope to win, everyone is battling the distance, not each other. This makes Ultrarunning a supportive and welcoming group. I don't know how much of this is the way the sport changes the people who take part, and how much is the way the sport attracts these people, but it is quite different from shorter distance running or even Ironman distance Triathletes.
The Zen. If you run far enough, you can reach a point of Stillness in Motion. This does not have to be Ultramarathon distances, but it has to be far enough to relax. This Zen like experience is one of quiet peace, with the world moving around you rather than you moving through the world.
The Spiritual. Someone once said "going for a run clears my head, but running 100 miles distills my soul", and for many there's a distinct religious aspect to long distance running. Some use their runs as an explicit part of their spiritual life, praying and meditating on the run, while for others it's a less formalized aspect.
The Second Dawn. Running through the dawn gives a sense of renewal as the sun rises and the world goes from darkness to light. If you continue to run through the day, through the night and on until the following dawn, then you experience the "second dawn". Running all day and into the night is tough, but the second dawn is a time of rebirth that cannot adequately be put into words and you will never look at the sunrise the same again.
The Shaman. It's rare, but there are some who run ultras to experience the shamanistic visions and hallucinations that can occur at the extremes of exhaustion. I've only come across a tiny number of runners who have mentioned this type of "vision quest" as a motivation, but it does occur.
The Kudos. Generally ultrarunners do not seek glory and are an introverted bunch, but there is some cachet to running extreme distances. I doubt if this is a significant motivation for the majority of runners, but it may encourage a few to try out a 50K if only to cross it off their bucket list. However, I suspect for most of us this is a nice bonus rather than a motivation per se.
The Freedom. Simply heading out on a run without constraints gives a delightful sense of freedom. This happens in the wilderness and it can happen in urban environments. There is a sense of chains dropping away and the restraints of everyday life dissolving.
The Solitude. Long distance running offers plenty of time to be alone, and many ultrarunners enjoy this solitude. It offers time to mentally relax, meditate and think freely. I am at my most creative when I'm running, and most of my writing is prepared on the run.
The Ephemeral Memory of Pain. The memory of suffering has some strange properties, allowing us to recall the outline of the misery but obfuscating the essence. Somehow we lack the ability to empathize with our prior selves, and so tend to discount the anguish.
The Indulgence. For those that enjoy your running, the opportunity to run all day can be a wonderful intelligence. I don't believe this is a motivation for people to run ultras, as most races involve pushing well beyond the point where running is enjoyable, and in to the territory of torment. However, it may be the reason why some of us do shorter races that are not so challenging. If you're trained to race 100 milers, then running a 50 miler without pushing the pace can be remarkably pleasant.
The Comparison. I think that people tend to judge their current mood based on comparison to the emotional highs and lows they've experienced. Therefore the distress of ultra makes ordinary life seem remarkably good. Even the stress of day to day training can make the remainder of life seem quite blissful. I have had many days where things have gone poorly, but that morning's training run where I experienced exhaustion, pain, extremes of temperature, and pangs of self-doubt, puts all of the days problems into a fresh perspective.
The Alternative is worse. However tough Ultrarunning is, for many of us the alternative is worse. Ultrarunning has saved us from something, and without it we would be far worse off. What the specific alternative is varies from runner to runner, but this is a theme you can often find once you scratch below the surface.
No Answer. The best answer is probably the least satisfactory: "for those who have to ask the question, no answer will suffice." The only way to know why we run ultramarathons is to experience it for yourself.