Local militia. That'd be the narrative reason for global polearm and light armour proficiency. I recently prepped a PnP adventure set in Elturel and each of the city districts maintains its own militia independent to Hellriders and Companion knights.
I'm guessing if I read up in detail about Waterdeep or BG that they'll have militias too.
That's far too narrow. A human PC could be anyone from anywhere. All humans are not militia. That was my entire point.
Militia could be a
Background that would give you those Proficiencies.
They need to solve the overlap with martial classes, who would actually use those weapons and armor in the first place. I can't really see Wizards or Rogues using Glaives just because they can.
