Some of the fellows on ODD74 were discussing the drawbacks and benefits of becoming the lich. In light of how "easy" it is to brew potions of longevity (3 weeks' work for ten years of life), it seems like sheer folly that anyone normal would want to become the lich! You could just stay alive indefinitely and enjoy many, many, many good years by drinking potions!
Of course you wouldn't want to become the lich. You are normal. You like life and would to have more quality years to enjoy it - even if a lot of the time is spent working. But that's only for normal people, like us and the ones you and I meet every day.
One of my meta-assumptions is that the race of Mankind is consumed by Ambition. That doesn't mean everyone is a maniac, but rather Men are more driven to excellence than are other kinds of people in the Realm. (This assumption explains a lot of things which I won't go into here.)
But even though Men are driven by Ambition with a capital 'A', not all men have the same ambitions and not all men have the same degree of - shall we say - mania about excellence.
Some people are consumed by different ambitions. They are held together by, their life-force completely directed by, some overwhelming negative drive. Without this drive, they would no longer exist in any real sense.
This second kind of person is the one who becomes the lich. Undeath is not a horrid fate to be avoided, but rather a blessing. No longer is he tied to humanity, no longer driven by food, companionship, sleep, or bodily functions, the lich is able finally to get its work done. And it never has to stop.
Liches are really, really driven to accumulate power. More than anyone else in history.
If there were lots of people like this, then there would be an arbitrarily large number of liches due to their arbitrarily long existences. In many campaign worlds there are zero, one, two, or a few, which means there are very few people indeed who have the power to become one, and also do indeed go through with it.
You or I would not become the lich, but that doesn't mean no one would. The lich is born long before the ceremony is performed to create it - lichdom is eternal, only marked by the softness of life at its beginning. Apotheosis is but the start of everything to come.