Given that this is the. major. problem. with raising money from institutional venture capitalists, it's not hard to see why someone trying to do lightweight capital for new companies would want to avoid that problem.
Remember also: they're doing two batches per year, the batch lasts 3 months, there's a lot of stuff that happens right after the batch, and the whole application/acceptance process is condensed down to a month as it is. There's not a lot of flexibility available in this calendar even as it is.
(Also: at least in earlier batches, there were IIRC companies that did negotiate the % YC was buying for their money).
The reality is likely that they are conveying their values through the investment terms. They want startups to be lean, fast, and high growth. Money makes you lazy. They're saying they'll teach you the actually valuable stuff in the program. YC also says they'll help you raise more investment money if you need it, which is true, I suppose...