Valve doesn't need to pay for exclusives because they don't need to entice devs to release a game on their store. Devs will do it because you would be crazy not to target their market.
Exclusivity, along with it's guaranteed income, and being promoted to a market where you are liable to be a bigger dog, is an incentive for a store that has a much lower market share.
Anyway, steam also doesn't need to pay for exclusives because they have their own tools that give them exclusives anyway. You people always seem to conveniently forget that steamworks exists. Yes, it is a more meaningful value-add than just paying someone, but often effectively means the game is exclusive to steam as well. It started with skyrim (if you bought it in a store it would install steam off of the dvd) and continues to this day.
All these companies, valve included, want the same thing. Customers to get stuck in their ecosystem and pay them for life.