Death saves

[stoutstien]

Logistics mostly. If I as the GM am running a combat with 3 bruisers as a front line, 15 skirmishers with bows and a couple of spellcasters to run interference, that's a lot of bodies and abilities to keep track of already.
Also running death saves for all those creatures adds a lot of logistical overhead for not much payoff.

That said, the game doesn't break if NPC's get death saves, you can absolutely run things that way if you like.

You can use a modified version of undead fortitude or relentless if you want to add some form of it for NPCs with less bookwork.