What is being called "Backstab" is getting advantage by physically walking behind the target, who can't move or react to that because it is not their turn. Anyone can get it, without any ally assisting, even without being a Rogue. This is being complained about because it is an extremely low-effort way to get advantage and the enemy cannot react to it to prevent you getting the advantage if you do not leave its threatened area.
Don't you trigger a reaction hit when walking around an enemy?
Nope. Only when you leave its threatened space (move away from it). You can move around it all day (that part is the same as table top).
You can also jump each turns to disengage as a bonus action... So you can very easily avoid reaction.
That said, an AOO is not enough to balance an easy advantage according to me.