What YC tries to predict is whether the founders are capable of making something people want, not whether the product they pitch in the application process is already something people want.
They are just looking for founders who can follow the build+release+test+repeat cycle as quickly and effectively as possible, and their claim is that if founders can do that really well in a large, valuable market, they will find success.
If no one is predicting that then the biggest thing YC is doing is making lots and lots of deals.