Transporting prisoners is a pain for PCs. It adds complication that's quite repetitive and not very exciting. Repeatedly surrendering NPCs typically don't lead to a fun game. Just imagine going through a dungeon with 5 encounters versus orcs and having to take prisoners in each encounter. What do you do with them? It just gets complicated. If the PCs don't take prisoners because of these practical concerns, then they probably won't even think of it as something to consider doing themselves.
The occasional surrendering NPCs can be good. Especially in an urban environment when the PCs have the resources to handle it better. And, the more NPCs offer to surrender, the more PCs will think of it as an option. Usually, though, they would rather die than surrender.
Trying to escape, though, I'm all over that. Things want to live. Chasing something down might not be worth it. If the enemy runs away, going after them might put you in more danger, you will use up more resources in your efforts to get them than the benefits of catching them. This goes for both PCs and NPCs. Lots of PCs won't run, however. Running can be difficult. Enemies will get free attacks while you withdraw, and if it doesn't work they're worse off than they were. Lack of real chase rules in the game mean there's little codified in the way of running. Spells make the best use of fleeing. Plane shift and teleport are the main ways of escaping from battles, I've found.
If everyone can't get away, though, it can be better to let people die if you think it won't be a TPK. Leaving behind that much treasure can be far far worse than the cost of a raise dead, after all!