Maybe too many Millennials grew up with Saving Private Ryan, where the released German soldier comes back and kills tons of Americans.
I've done that once, with a particularly nasty ogre; but normally IMCs enemies who flee aren't keen on a rematch, and surrendered enemies are often recruitable - my son is particularly keen on doing this, and it's extremely Gygax/Arneson Old School, much moreso than always killing everything IMO. There are various ways to make enemies worth more alive than dead, eg ransom, which is standard in Runequest and should be standard in medievalesque settings with feuding nobles. The more you get away from 'hostile races locked in a war of extermination', the more not-killing can be normalised.
Edit: Killing is ubiquitous in computer games because it's a lot easier to code than enemies who surrender. This is definitely an advantage of TTRPGs.
I think a lot of us have had somewhat formative experiences with the kind of GM (or at least session) that
@BrokenTwin recounted, where something like letting enemies go is seen is a lack of proper grit, which must be conditioned out of us silly, naive players. If that happens even once, or even if you just have the tiniest question about whether your current GM might have that sort of a mean streak in them, I think it could easily become part of the unspoken rules-of-roleplaying that we tend to codify for ourselves.
I'd also propose a couple more factors that make capture/surrender unappealing at lots of tables:
-So many prisoners: If surrender or capture is an option during the second fight of the delve/crawl, and everyone basically knows there are many more fights to go, then the logistical weirdness of taking prisoners is only going to get worse, and if you let the enemies go, why wouldn't they just warn and add to the numbers of the next group of enemies down the hall?
-Don't talk to the prisoners: In addition to stuff like figuring out how to secure and maybe transport prisoners, you kind of have to talk to them. And if they're just one of many, many enemies you're going to fight in this situation, or if you're just getting in fights all the time, this could start to be a real roleplaying chore for everyone, PCs and GM included. Even just the prospect of playing out yet another gnoll's tale of growing up poor in Menzoberranzan would have me pushing them to fight to the death.
To me, the obvious solution to all of this is the same thing that I think improves most games, which is way, way fewer total fights, and giving the ones that happen--or that you actually play out--high stakes. Taking a couple prisoners or letting someone go isn't all that weird or a hassle if you only have a fight every two or three sessions, and there's a greater chance that any interaction with a surrendered enemy will be more meaningful to the narrative, and less of a repetitive drag. Plus, if the fights are generally higher-stakes, maybe the PCs are more often fighting through to get a way from a situation, and not just having yet another in a series last-man-standing, OK Corral-style standoffs.
But if I were running something with one or more fights every session, I'd probably wind up making every enemy a mindless drone or total fanatic. Otherwise, that prisoner small talk would be lethal