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.
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.
Yup! Then try hitting the wall at 600 mph, after raising $100M. That ain't pretty either.