It's for economy of scale for presentations, demo day, etc. Also to make it easier for YC to keep track of what stage people are at - i.e. all 3 weeks in, etc.

It's for economy of scale for presentations, demo day, etc. Also to make it easier for YC to keep track of what stage people are at - i.e. all 3 weeks in, etc. Having people start any time creates more administrative work.