system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why we can't, say, have 1000's 
of NPCs fighting at the same time in 
Skyrim, but we can have, say, 1000's 
of zombies in L4D2?

L4D2 = Left 4 Dead 2 for those who don't know the game.

I've been wondering this. I mean, they both have to deal with pathfinding, animating actions, choosing who to attack, etc. So how come I can face a horde of 1000 zombies in L4D2, but having a 1000 NPC war in skyrim is not quite feasible for PC's yet?
gpt
It's not about the capability. They're very different games and very different designs. L4D is built around fighting a "horde" of dumb unarmed enemies Skyrim is not and so was not built with this type of combat in mind. 

On a much broader level the zombies in L4D are very very simple and just follow paths to run on then have a single simple attack. A game that involves massive amounts of enemies that might fire guns, bows, use magic, swords, etc. are much more complicated and it wouldn't be fun to have 1000 enemies with projectile magic or even just swords attack you all at once. 

Short version: It's a design limitation not technology.