I'm sure they looked at doing it. The UI bouncing up and down as notifications came in could have been really annoying for internal testers. People need to "trust" the button placement on touchscreens so moving it constantly would screw that up. And you'd also have people complaining that just when they are about to hit Back a notification comes in and the button moves.
I think the current implementation is the lesser of two evils.
I think the current implementation is the lesser of two evils.