Because at least at the startup you have the possibility, however remote, of getting FU money and never having to answer to anybody you don't like, ever again.
And besides, if you're that good, you don't really have any worries about getting paid what you're worth.
The whole FU money is mostly a fallacy. If you really love filesystem hacking, and you live in Silicon Valley and are really good at it, you already never have to answer to anybody you don't like, ever again. If you don't like your boss, you can quit, and do this 50 times if you want.
And people shouldn't work for a "possibility, however remote." That's called playing the lottery and is generally foolish.
And besides, if you're that good, you don't really have any worries about getting paid what you're worth.