Some are, but a lot of them are also made to be successful long term companies. Small business is still a large employer overall. Some businesses need to be very big, and if you want to be "filthy rich" you have to be in one of those. However you can be nicely wealthy at a small company.
I still have a hard time understanding what's going on with startups : the power of money makes it too attractive to sell and "call it a life" I guess. That's why even if they don't plan on doing this, startup might actually accept being bought. I guess.