[meowface]

As another commenter said, this is just financial opportunity cost. You can consider it in reverse: imagine the inspirational/motivational opportunity cost incurred by working at a megacorp, when your soul could feel far less deadened by working on a project you have total control of.

Of course, if someone is working on a startup just to get rich, this won't be the case, but I think this isn't that common, or at least isn't in the majority of cases. You only get one chance to exist, and it's infinitesimally short and fleeting; why not spend it doing something that maximizes your enjoyment of it, rather than your financial reward, if it's at all possible to do so?