Web games benefit significantly from SAB (see the Godot engine for example) and mobile games make up a pretty sizable chunk of app store usage, particularly in app purchases.

We can't say that for sure. There is no shortage of examples where Apple neglects a feature that might provide parity with their own services to avoid competition. Safari, being a mandatory feature of iOS, is reasonably implicated as part of the conspiracy to prevent users from buying and selling software without the assent of a corporate benefactor.