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.
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.
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.
The issue isn't that high ground gives benefits, the issue is that granting advantage on attacks for and disadvantage on attacks against is overpowered. Advantage and disadvantage are specific mechanics in DnD that are more than just increased accuracy, evasion. OP even brought up that they're okay with a +2 to hit, -2 to attacks against. Attacks outside of effective range doesn't contradict OP at all, it's already in the game.
IRL in a sniper duel, if a sniper misses their shot they will always change positions because they have no protection. High ground or not they have been given away and the opposing sniper now has an easy shot. Castle walls had trapezoidal openings so that archers could be protected from incoming attacks. They weren't just standing out in the open on top of the castle, opposing archers could take them out too easily if they did that.
The high ground doesn't protect you from ranged attacks IRL, it forces the opposing team to exhaust more energy/resources climbing the hill, sieging a caste wall, etc. Baldur's Gate 3 doesn't have characters expend nearly any energy for going up and down a ladder. So the whole economy of high ground benefits doesn't make sense.
Originally Posted by etonbears
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.
But somehow boxers train in learning how to uppercut rather than punch downward. Being able to rotate the human body in 3d space easily overcomes the kinetic expense of punching upward. And it can be strategically worth it to punch upward. Also if kinetic penalties really stopped someone, no one would go for high knees in MMA.
Again we're all okay with high ground having benefits, but advantage for / disadvantage against is ridiculous. IRL if the enemy started with higher ground you would find cover to fire safe shots from, not run to high ground yourself.