Many reasons. Perhaps they need guidance on how to develop the product or how to scale. Perhaps they need funding. If you're asking something like "why give up equity when you're already making money?", I get the sentiment of the question but a) you don't know if they're profitable and b) if YC can help increase the value of the company significantly, well then 100-x% equity might be worth a whole lot more than 100% if the YC investment and involvement ends up being accretive.

My point is that those basic stats don't really tell you anything about whether joining YC makes sense.

Not OP, but from experience, entrepreneurship can get lonely and it's nice to be in a cohort.

I've been in several with different startups and the low quality cohorts can be quite demotivating. There are several groups by self-made entrepreneurs and a good bunch of them are sociopaths.

I'd say YC is nice because they genuinely care about making a positive impact on the world.

I wanted the network-- not the money.

some yc applicants already got seed funding, reasons could vary from exposure to connections etc