Multithreading may be an obscure feature to you but runtime developers get requests for it all the time. SAB becoming widely available was definitely delayed.
Did use cases exist? Sure. But not sufficiently to move the needle on app store usage.
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.