You do make a very good point that it is very cheap for the strongest AT gun in the game.
And it will Vet fast because it is cheap, shooting at tanks and doing tons of damage, so even if it does get de-crewed it's not the end of the world.
And as Hector noted, whilst it can be hard-countered by munitions expenditure the enemy is spending a few hundred Munitions to counter a 500 hundred MP unit.
That is not necessarily a favourable exchange for them and if they are bombing that they are not bombing anything else.