I'm going to take a guess here. YC doesn't have to give up the best of its limited resources to competing startups. They just help get you to a Series A round

I'm going to take a guess here. YC doesn't have to give up the best of its limited resources to competing startups. They just help get you to a Series A round. They can do this for competing companies without any issues. VC's help assemble an executive team, provide critical business relationships, etc., and it's not in anyone's interests if they have to divide their best contacts and resources between two competing companies. They should want to go all-out in providing one company all their best resources.