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.
What other things you need to pay attention to? Other then ones you need to pay during the fight itself?
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.
Boxing, mma and other such sports take place in a very confined arenas -for a specific purpose of preventing disengagement. However in real fight where there is no space limit, there really is no way opponent can hit you if you decide to disengage and flee. Only option there is for him is to try to run you down. If he’s faster that is. But unless you do something stupid, like deciding to flee in the middle of his attack, there’s no way he can land a hit on you simply because he doesn’t have a reach and given you have initiated the action, you will also have a lead on him.
There are few specific situations I can think of when above might not be the case, like for example if guy with a knife faces guy with the spear and he managed to close the distance to the knife range. But in that case he basically won the fight, spear posses no danger for him since he is beyond inside effective range of the spear and it’s the spear guy which should think about running away. Or at last increasing the distance again.
In most other usual situation, fighters position themselves just at the edge of the effective strike distance of the opponents weapon (might be fist in a fist fight). Which is what basic stance one tile away from an enemy in the game more or less represents (obviously in very simplified form). Initiating move away in such a position is very safe, unless you trip over or do something incredibly clumsy.
Now I am obviously not talking about situation when two opponents are grappling each other. Then moving away is not an option unless you ungrapple first.