If you fly really high up, then drop down and go at nearly the speed of sound, and then hit a tank with the plane... you'd expect the tank to be destroyed too, right? But it doesn't. What gives?

I dont think the game counts your plane itself as an enitity that can damage tanks... im sure the game thinks your plane is an enitity that is capable of shooting bullets that damage tanks but cant damage tanks by ramming itself into one.

Yeah you're not playing that game to suicide into other players. It would ruin all the fun