[danbmil99]

From a market point of view, founder of a startup is unlikely to have lots of opportunities to be CEO at other companies. The kind of person who comes into an established company and takes the CEO spot has much more leverage because they are a generalist who could be CEO at a bunch of different companies. The founder is a specialist in their own venture, but doesn't have much market value outside of that role. And of course the founder presumably has a large chunk of stock and therefore has an order of magnitude more upside than anyone else who might run the company.

Finally, there's the fact that psychologically the founder often believes they know best, and has a higher level of expectation for the company's prospects than a cold-eyed third party who wasn't part of the founding story.