Still, it’s easy to understand why retreat is so undervalued. Lots of systems actively aim for really swingy combat, where one side or another ends up dead before retreat can really even be considered.

Nah, I’d say it’s usually on the players’ egos more than anything else. A decent player can tell when a battle is not going well within a round of two, even in something fairly brutal. But many players don’t think about running away, viewing a combat as something to beat and “win”.