YC takes 7%. Lead seed investor wants to take 15-20% to make it worth their while. Lead series A investor wants to take 15-20% to make it worth their while. Other investors and option pools each round increase dilution further.

Founder doesn't want to give up 50-60% of company by their series A, so because they gave YC 7% they must either convince the lead investors to take a smaller percentage, or convince YC or seed investors to drop their pro rata rights.

Of course, you could argue that the company wouldn't have seed or series A investment had they not joined YC, so in that sense it's a nice problem to have.