Or similar to why engineers get stuck in managerial roles.
The money stops at a certain level, then they become a manager to reach the next pay grade. Then don't enjoy their job because they're not doing the engineering, but can't go back to the lower salary once they've adjusted to the lifestyle
Also many of the founders have an "entrepreneurial spirit", which really means to me they prefer to start things, make it a success (or not) and then move on, over and over again. They are not often a "lifer" that hang on at the same company till they retire. Some are, many not.