I think there are a lot of games with this (or similar) mechanic. From Dungeons and Dragons, to BloodBowl.
From a realistic point of view, when you engage, you are prepared: you are paying attention to your opponent, and so. When you disengage, usually you need to pay attention to other things, so the oponent have an oportunity over you.
Perks like footwork or rotation represents the hability of a character to pay attention to the opponent and whatever else needed (as terrain) to disengage safely, or fake an attak while really disengaging.
If you see any fight-sport (boxing, mma, whatever) you will notice something like this actually happens: when engagins, both cmbatants land their hits, protect themselves and maneuver in safe. If one try to JUST disengage, his opponent gets an oportunity to land a hit or two.
In the other hand, in the game is a feature that favours the melee and thinking about your plan before you commit it. And there are mechanics so you dont get hit, even when you dont have a dedicated perk for it (like footwork). If you want to disengage safely (like in real live), try to rise your shield or push away the enemy.
