Ok, on a sportscar, you have maybe 1500 lbs of weight in the car.
Now they know guys are going to use and abuse them a bit, so they overbuild it to a point.
The other thing is your limited to 4 wheels being your only contact with the ground.
With a sportscar on pavement, worse case scenario the tires break contact and skid.
With a grain truck, you have 60,000 lbs or more on 6 or more wheels.
Trust me, those wheels will NOT break contact. So instead of spinning the wheels it breaks a drive shaft, or throws a gear.
With Tanks its even worse, you have tracks. VERY difficult to "break contact" with a track on almost any ground other than soupy mud. They just don't do it, combination of weight and "grip" just doesn't let that track spin like a tire does on a sportscar.
So, instead of being able to slip, its going to throw a track (too much stress at one point, will sheer the pins)
Break a drive shaft, or turn steel gears into confetti.
Yes the gearbox works basically the same way as on a car.
But you don't have the safety built into it like a car does. (can't slip wheels)
So if your even a little bit off on your guess of which gear, and what throttle setting.
Instead of just squeeking your tires, your going to cause major damage to the drive train.
Hence its just not worth the risk.
That's not to say trucks don't downshift, they do, good ones do it a lot.
But they are not rolling down a hill at 20+ mph, drop it into a gear and pop the clutch.
Because that's a recipe for disaster.
And even more so for tanks than trucks.