And human beings get chased down and killed from behind throughout history when they tried retreating as well.
(bold added) You wouldn't in my games.
You said yourself you set up your scenarios to allow PCs to escape, etc. I don't. The enemies (if intelligent at all) strive to have the battlefield as much in their favor as possible. I don't favor the PCs at all. They have to find a way to work things out to favor themselves. If the players are at a point where they need to escape, often they will no longer have the resources needed to help them escape--they will have exausted those options.
Some of the things you describe in your post upthread seem to imply things that aren't quite RAW. How your enemies behave is also an issue from your limited descriptions. If the enemy has the upper hand, for example, and a lone PC runs over to threaten the boss, why would the minions run to the aid of the boss when they could swarm the other PCs, take one captive, force them to surrender or watch their friend die, etc.?
The two examples I posted show how in D&D attempts to retreat will often lead to failure. Unless one side or the other can really out distance / maneuver the other side, the cyclical nature of initiative means the chasers will usually get at least one attack per round against those they are chasing.
Just running away rarely works. Distractions, etc. like you suggest require actions to implement, and if you are already engaged with the enemy, that means you really aren't using your actions to get away, and they continue to attack you.
It is very much a rules issue... as most people have posted. RAW does not easily allow escape--you absolutely need something (cunning action, magic, etc.) to help you get away OR you have to be in a position to out distance and out maneuver your pursuer.
The very fact that you "do it all the time" simply demonstrates to me a difference in game style more than anything else. How a BBEG could escape with a simple fog spell, for instance, screams of a situational thing. How big was the encounter area? How close were the PCs? How large was the area of the fog? If the PCs can't see the BBEG, how does the BBEG see to escape? And so on...
And how "clever" a player is I suppose is fairly subjective. The way you express it I find a bit off-putting. It almost seems like you are saying "if you can't find a way to escape, you are not clever."