Valve has like five games, and the hundreds of thousands of other games on Steam are from other companies, and they’ve had 3rd-party games since 2005.

Activision/Blizzard didn’t put their games on Steam so they can push their own store, it was to not make payments to a third party, and have high visibility of their own products (e.g. advertise CoD to Diablo players and vice versa). Of course, they miss out on the visibility of being on the largest game marketplace.