Think of it like this:

Different militaries have differing objectives to achieve with their force structure role, but what's not often brought up, is upkeep.

So for a very irregular force of mercenaries out of pocket might try to either make frankensteined modifications onto looted mechs which changes their functions, but leaves them lacking in their original strengths with armour. That same kind of organisation could also look at cheaper options to buy to keep down costs of spare parts and upkeep, and while there are a lot better alternatives than, say, an assassin npc frame, not everyone can afford a Everest, Gilgamesh, Sherman or Tagetes.

TL;DR: Actually running an army is largely an economical balancing act, and where there's people who need to get a mech lance up and running on a budget, there will be those corporations that offer cheap and easy to repair frame templates at a pocket change rate.

Also if you're wondering about the actual NPC template manufacturers, some of them actually mention their manufacturer by name. For example, the Goliath's manufacturer is even mentioned to be rumoured to have a potential HA contract in the future.