wdym? Try to imagine your campaigns in a real d environment, your archer is going to want to go to the top of a tower or climb a tree for advantage right? If your rogue sneaks behind someone hes gong have a good chance of dealing some damage. Just because the environment plays a part in advantage does not mean spells are useless. You wont always be able to take the highground without investment.
Archer is gonna go high-ground for advantage, but not the advantage in terms on DnD rules which is much stronger condition (again, see first post). By being high-ground you can ignore ground obstacles, in other word, you can ignore eventual cover. And don't forget the high ground advantage is currently applied even to spells like Guiding Bolt, which is by description a beam of ligh. How does a beam of light take advantage from high ground?
We have no problem here with rogue sneaking behing someone to get extra damage with good accuracy. The problem is: (1) everybody can do that, not just Rogue. (2) It' not sneaking when you leap over enemy or walk around them in middle of fight. (3) You actually don't even need to succeed in Sneak skill check. (4) You can backstab with spell also, try e.g Inflict Wounds.
So, I actually agree what are you saying but the game behaves very differently.