Because hedges obscures line of sight, and that makes perfect sense. This is a matter of projectiles being poorly animated/programmed to collide with the ground every time it tries to shoot at something elevated. If the tank with high ground advantage was slightly more accurate, fine with me. In no realistic scenario would someone nail the ground in front of the target every time because they can't compute a difference in height.
You have to consider what is between you and the enemy unit at all times in this game. It's no different than how with infantry combat you have to think about cover between you and the opponent.
You know that being on the low ground means your shots are more likely to not hit your opponent. Therefore it becomes a part of your tactics: controlling the high ground means you are better protected, while attacking from low ground will be much harder.
You have to stop looking at the game with an a-move mentality. The best part of this game is that where your unit is standing is actually fucking relevant to how it performs. If you position an artillery piece next to a house, it might hit the house instead of where you wanted it to. If you try to throw a grenade, it may get caught up in buildings or hedges in its path. You have to think about these things all the time in this game.
Also, tanks generally don't have much ability to adjust their aim in terms of verticality. A few degrees of pivot at most. Not to mention that units on high ground are literally more difficult to hit with any sort of ranged weapon and that has been true for thousands of years or warfare whether you're using bows, guns or tanks.