The producers sign the contracts with the people they meet in person to make the videos, so they can verify those same people signed the contracts. A web site has no way to check if the contracts are real. People can just upload made up documents.
So you think showing NO proof is better? It's real world we are talking about. The reason backpage and co got busted by the FBI is because of the amount of shady activity. And pornhub is definitely hight on the list of shady sites. Also a lot porn site are just own by porn company so they already have the contracts. Just don't be surprise when you see FBI indictments.