And using the gun depression argument isn't valid since a tank on a hill would have an equally hard time (if anything a harder time) depressing downward versus shooting up.
You would be correct if hills were flat plateaus at the top. See exhibit A below.
Also you have to consider where the strongest armour on a tank is located. A tank on a hill has its best armour facing the opponent, while the opponent on the bottom has some of his weakest. (Second only to the armour underneath)
The advantage of high ground is that you are a much smaller target and harder to hit, while you have full view of your target from up top and they are very exposed. (In melee fights you have leverage, but that's not applicable here)
I realize that some maps have slight slopes that cause issues, but so will any sort of cover or debris in your path. As Someguy has explained, if your tank is hitting the environment *it already missed its diceroll*.
Lets not make this game into SC2. Every diceroll is a risk/reward scenario that you have to consider. One of these is that you have to be mindful of the terrain. It'd be an issue if it was only one unit, but it is very consistent behaviour for nearly all weapons ranging from AT guns to tanks to bazookas.