Dude you potentially miss a lot of opportunities when you filter on the basis of having a cofounder. PG claims a startup is too much work for one person. duh. Thats why you hire employees.
Well, it's their money, they do as they see fit. If some other, similar seed-funding group accepts lots of single founders and does better than YC, maybe they'll change.
It's not just them saying this, though. I really wish I had someone to work with myself; it would help things a lot.
And things are never all else equal.