Why? Because the game would be too easy without it. To fix it being too easy, they would have to change more than they would be willing to change.
The things they'd have to change are mostly things that should be changed anyway. If things like disengage and hide were actions for most classes (exceptions being rogue and monk) and height advantage couldn't be so easily abused, combat wouldn't be as easy as you seem to think. Especially not if creature AC was made compliant with the MM.
