Not to mention that when you run after a fleeting opponent, you’re removing yourself from the rest of the party and combat, which have consequences of its own.
« You can never escape » is an hyperbolic and weird take indeed, but it’s rarely a done-deal either. Flying creatures can expect one or two rounds-worth of attacks as they go, potentially with cover if they’re smart, but still… Most teleporting abilities are short range and the equivalent of a move, sometimes a dash action. Many invisibility effects will only get you one turn, which is sometimes enough but not always. But when a creature is about to flee, be it PC or enemy, it’s usually because their ressources are getting low (in hp and abilities). And when the creature decides early on that fighting is not a good option and flees, their opponents are also high in resources. So unless they are not willing to spend these resources to pursue (which should happen more often than it does), the escape is far from guarantied.