It is because the core 5e follows restrictly the design of bounded accuracy, and adding more tactical aspect to combat. You need to shove someone prone, or use spells like Faerie Fire to get advantage, not brainlessly walk to someone's back since advantage is very very strong. While here it's just free for melee attackers. What you said about using surrounding and circumstances is valid for high ground, but not for backstabbing. You DONT need surrounding or circumstiance to get backstab, you JUST walk behind a dude and attack him, no AoO, no resource, just free.
That's not entirely true, at least for movement that starts within the AoO range of the enemy. If my Rogue is standing right in front of an enemy and uses Jump to leap overhead to their rear for a backstab, it will trigger an AoO attack from that enemy. Same thing with a fighter who starts within AoO range and just circles around to the rear of the enemy for advantage. That also triggers an AoO attack. I see both things frequently in this game.
The problem right now is movement starting outside the enemy's AoO trigger range, and then entering that range while jumping or moving around to the rear. That should trigger an AoO and it doesn't. The only "free" melee attack from the rear should be when taking a path that starts behind the enemy, approaching from the back.
