I think the issue may be that it would then need yet another identifier for calls.
What if you pressed that button while you were in the car and connected to CarPlay? Or in a room and already connected to HomePods?
Do you ALWAYS want to make that call via speakerphone? Maybe you do, but I’m guessing it is more complicated when folks want that shortcut, but only when the phone isn’t connected to another audio source.
But this is coming from a guy that is too slow to have caught on the shortcuts yet…so…