Because you want to start a business that supports itself with profits instead of outside monetary investment. Because you want to build a great version of som

Because you want to start a business that supports itself with profits instead of outside monetary investment. Because you want to build a great version of some X but don't necessarily want to disrupt or reinvent an industry. Because you want to be financially secure and not risk your financial future on an idea that isn't guaranteed to work. Because you like traveling or taking time off or playing around with a lot of different ideas and aren't at the place where you want to commit to working full-time on a company for five years. etc... This isn't a knock against YC at all, there are just other ways to grow and scale a business than hyper-growth + raising outside capital, and sometimes even if you have a great idea, bootstrapping can be a better approach for you depending on what kind of person you are and what point of your life you're at.