The only reasons I can imagine for a creature having advantage with ranged attacks on the high ground are:
1. If their body is partially covered
2. If their body is obscured by sunlight (but in order for this to ever happen the devs would need to implement the day/night cycle; something to consider).
There are many advantages to having the high ground in pre-modern combat, which is why almost every respected military authority in history recommend seeking high ground, or at least avoiding the enemy on high ground.
Ignoring the psychological benefits and greater visibility, the primary benefits are physical.
When a ranged weapon is discharged, it immediately begins to lose kinetic energy through air resistance, and as the speed reduces, so does the accuracy and terminal hitting power. The 5e rules ALREADY use the advantage system to impose disadvantage on ranged weapons after some notional "effective range". ( This is in direct contradiction to the OP who inaccurately insists that the advantage system is never used for mere movement/positioning ).
If you have high ground, your ranged weapons receive a kinetic benefit from gravity, and conversely, if you are firing up towards high ground, your ranged weapons receive a kinetic penalty. As 5e is such a simplified system, the advantage/disadvantage system is the obvious way to reflect this. Particularly in a game where just about every environment includes significant verticality, not to reflect it would be perverse.
Similarly, hand-to-hand combat is significantly more difficult if you have to consistently wield your weapon in shortened arcs above shoulder height, and much easier and more effective if you can swing downwards with elongated arcs.
Of course, not all ranged weapons, and not all melee weapons are equally affected, but within the bounds of the trivial nature of 5e rules, and the focus in BG3 on height, using the advantage system doesn't seem unreasonable, particularly as it gives martial classes more reason to exist.
You may notice that I did not mention spells. This is because there is less obvious (+ve or -ve) kinetic effect with most spells, so less real reason that spells should be affected by height.
You can obviously argue this any way you want, but the OP arguments as concerns height are particularly weak.
The "backstab" situation is less easy to rationalise. I would expect the 5e rules to have some "zone of control" mechanism, but there seems to be nothing apart from attack of opportunity. I've seen some arguments that circling an opponent within weapon range should trigger an AOO as you move out of the front/side arc to get the backstab; this might help.
The real problem though, is that the tabletop, where you typically move characters via pick-up-and-place-down, will ignore things that then look silly in a videogame where you show continuous movement ( ignoring the unrealistic serialisation of movement inherent in turn-based games, of course ).
Maybe we will get something from the "Panel from Hell 2" in a few days; maybe WotC will even add some 5e rules/clarifications to resolve the argument.
