NPCs have different design parameters than PCs. PCs are the stars of the show and get all the “stage time,” but a typical NPC is supposed to be an obstacle for the PCs and is expected to get killed after one combat–a combat that typically lasts 3 to 6 rounds.
So if you’re building an NPC barbarian for a combat encounter, and this barbarian is not someone important to the story (in other words, they aren’t going to retreat from battle and return later, they’re just going to fight until they die), take a look at how many rounds of rage per day they get. For example, a 2nd-level barbarian with 14 Con has 8 rounds of rage per day. That’s more than enough to last a typical I-am-here-to-fight-and-die battle.
In that context, there’s no reason to give that character the Extra Rage feat (which adds +6 rage rounds per day) because under most circumstances, the character isn’t going to live long enough to use all of its rage rounds even before it gets to the extra rounds from Extra Rage. You’d basically be wasting a feat on something the character would never get to use, just as much as if you’d given this combat-encounter-doomed-to-die barbarian Skll Focus (Appraise).
(It gets even worse if the barbarian is a high-Con monster with barbarian levels, like a hill giant barbarian. A hill giant barbarian 2 using stats out of the Bestiary has 10 rounds of rage per day. It doesn’t need Extra Rage. And that doesn’t even take into account modifying the giant’s ability scores with the elite array… if you put its +4 into Con, that same barbarian starts with 14 rounds of rage per day.)
I’m not saying don’t give NPCs things they can’t use in combat. Obviously for a recurring villain or other character important to the story, you want to have well-rounded characters and represent them with appropriate skills and feats for that character’s background. I’m talking about nameless “mooks” that are just there for the PCs to beat up on a bit before they get to the real boss (like “the PCs are ambushed by 4 2nd-level barbarians”). Considering that fights are already stacked in favor of the PCs, you don’t need to go out of your way to hobble the NPCs for those combats. Basically, Extra Rage is mainly a feat for PCs.