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.

Maybe because killing ground and air targets awards you points, and if ramming a plane doesn't give you any points why should ramming a ground target give you points? Imagine a plane with no ammo and on fire about to die (being the only guy left) rams a tank destroying it and winning the game? It would be unfair and stupid

sometimes, if you smash into a truck, you will damage it or even destroy it, but I rhink you need a quite big plane.

Yeah that is quite stupid, when I was a rookie I tried doing it and it didn't work out.

because war thunder doesnt have good mechanics

Nobody knows

Yea i tried it many times didn't worked but for vehicles its working :)