One reason for this is that they're working with tens of companies per batch, and the way the application/interview process works doesn't give them a lot of time to wait for people to accept. If you make the deal negotiable, people will try to negotiate.

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).