First, there's a difference between the type of person who can build a successful product and the type of person who can run a successful business at scale. These two characteristics may reside in a single person, but they don't have to.

There's also a lack of excitement inherent in running a company that's no longer a "startup."

There's an exchange of current capital for risky future earnings which may be attractive.

There's burnout.

Perhaps there are more reasons than that, but that's what I can think of.