I think elevation giving advantage only makes sense.
Also positioning benefits are in rules. Page 252... So I am not even sure what exactly you're so offended about with Larian using the rule.
1. They didn't actually implement that rule (only the Advantage part and not the other 90% of the optional rule)
2. No one uses that. Some DMs use Flanking, some don't. I've literally never seen a DM use Facing.
3. They literally used the same mechanics as "Backstabbing" in DoS 2 but didn't include the ground indicator to know if you're going to get it or not.
I lied, 4 reasons:
4. Advantage for positioning yourself behind someone is drastically overpowered, and it cheapens other sources of providing advantage. This gives melee combat an extreme amount of "sameyness" where the objectively best choice, each turn, is to circle-strafe your opponent to get Advantage, then they do the same to you.