Except asking a bus driver to keep track of button pushing is distracting and could endanger the lives of the passengers or people outside. The stop needs to be announced 30-60 seconds before they arrive at the stop so then is driver is still actively driving. This is something the drivers union would be all over, it's not as straightforward as you assume.
If there's a long gap between the stops, people may forget what's the next stop. Announcing at the opening may be too late or the stop has to be long enough. Even for slow passengers in a crowded bus. Announcing a minute before the stop is a good heads-up that bus is approaching the stop and people should prepare.
