The fixed cost of hosting a game is relatively fixed (providing a storefront, first level customer support, license management), and the part of the cost that scales but is still cheap (bandwidth is larger for bigger titles).

However, there are many times more small indie titles than there are big successes, and it's the big titles that draw users to your platform and bring in the money.

If Valve flipped their model, those indie titles would be even more of a net drain on their profitability, while there would be more incentive for the big titles to release on their own platforms.