Well, apart having personal advice on tap from a range of experts, the fact that on demo day your idea will be in front of the best VC groups in SV, access to YC alumni for help and advice, I can't imagine why people would give up 7% of their nascent startup either!
(this is sarcasm btw.)
More seriously: you're giving up 6-7% of your company in return for a whole swathe of things that are very likely to increase your chances of succeeding significantly. Most people don't have ready access to the kind of personal networks that YC offers: the combination of that access, plus the social proof of having the YC imprinteur massively increases the likelihood of success for many, many startups.
And I'm saying this as someone who's very unlikely to ever apply to YC, but even I can see the benefits in applying, as do many others who choose to go to YC even if they don't in fact need the money: the fringe benefits are worth far, far more than the cash.