Busses and trains aren’t good targets for terrorism.
You blow up a bus, maybe no one really cares except the locals to the area where the bus blew up.
A plane cuts through many different social classes and nationalities, putting them all in one place and making a large amount of people imagine themselves as the potential victim.
But if all flights were completely unscreened? It wouldn't change what or when I fly.
I already board unscreened trains and busses quite often.