There are plenty of successful founders who didn't quit their jobs immediately to work on their startup. Not everybody has that luxury. Maybe you meant to limit your comment specifically to YC startups?
I partially agree with Pytrin. While its true not everyone quits their job to dive into a startup immediately, its arguable whether or not those are even startups yet or just projects.
That said, if you aren't at the stage where you can commit to doing a startup, you probably shouldn't be applying to YC.
I wouldn't say plenty - succeeding while working on a startup part-time is definitely the exception, and not the rule. I would love to hear some examples of those successful founders who succeeded while working part-time on their venture.
That said, if you aren't at the stage where you can commit to doing a startup, you probably shouldn't be applying to YC.