> What evidence? My account is full of games (including top sellers) with no such arrangements.
Ignore indie games that have been on Steam for years and years, or that only get published on Steam and no other platforms; these are the exceptions to the rule (despite this set containing some of the largest hits by sales volume.)
While there are studios that sell only on Steam and other low-barrier-to-entry channels, 99% of them don't last more than a year or two, because selling only on Steam is leaving almost all your money on the ground. There's a reason that many of these games don't get support updates any more and won't run on e.g. macOS or Linux after any major OS update, despite originally intending support for those platforms: the studio didn't survive.
And while there are indie studios that eventually take their console-exclusive game over to Steam, it's often still published by the publisher on Steam. Take a careful look at the Steam catalog page for the "publisher" field. If there is one? That's who's making the direct revenue on the game sales. Like the publisher of a book. The "author" — the studio — is only getting a commission.
There are a few indie studios who manage to "earn out" their deals with publishers, and take over their own Steam pages (though not usually their console marketing rights — the platform owners don't like dealing with the long tail of self-publishers, they much prefer well-known bigcorps as marketing partners.)
