I don't buy someone lunch with an implicit expectation that they'll buy me lunch in the future. That's tacky and gross. I buy lunch because I wanted to buy them lunch, and if they decide to buy me lunch, I happily accept.
Right, all the games that they think will be successful. Most games won't- it's a power law market.
There's nothing preventing a game dev from selling exclusively on their own site. It's not as though Steam has exclusive access to Windows customers like the App/Play Store do on their platforms. Steam earns its customers and their trust and developers follow.