Well, why did you ask what games have exclusivity contracts with Valve?
My point was, Epic creates these exclusivity contracts for games they fund. I think it's a net positive for there to be more art in the world, even if you have to wait a few months to buy them from your favorite storefront.
Valve doesn't fund the development of third party games. They do create some games in house (fewer than they used to sadly) but just like in Epic's case these are exclusive to Steam.