I'm not sure if tanks have throttles such as aircraft do, but certainly for trucks like the M16 or M3, I can think of a perfectly good reason: They don't have throttles, they have gas pedals which require constant pressure to get power. Mapping it to a stick axis makes a little more sense than a throttle to me. Besides, if you want to maintain constant speed without having to push on the stick all the time, you could just switch to a gun view and steer with rudder, which is what I always do. You can see better.