Not just that. Defining a successful startup as a startup that exited is a bit narrow of a definition. A bootstrapped startup that has sustainable growth and can remain independent is successful in my book.
A bootstrapped startup that you stop paying attention to will rapidly stop making money.
Yup! Then try hitting the wall at 600 mph, after raising $100M. That ain't pretty either.
For some, sure. For many others it's to have the freedom to do something they enjoy, or to solve a pain they've encountered, or to help others. YC funds plenty of non-profits that aren't likely to ever make the founders "fuck you money" rich. In fact, most of the founders of those startups could earn much more elsewhere.