Why Hire Adventurers?

[Grim Portent]

There's always going to be a point between "a threat is worth wiping out" and "that threat is actually wiped out", and a mechanism by which that threat gets wiped out. It's not really unreasonable for "adventurers" to fit in there, especially for things less mundane than "a tribe of goblins". If the problem is that a cult has set up shop in the woods outside town, it's pretty easy to understand why that won't necessarily correlate with existing borders. Nurgle (or Orcus or Ruin or Cthulhu or whoever goes around setting up cults in your world) doesn't have a material kingdom he sends out envoys from, he just shows up in people's dreams and tempts them with promises of power.

That's much more the sort of work that should be getting done by witch hunters, an inquisition or similar semi-centralised agency of government, not a bunch of random people who happen to face peril for pocket change.

The issue for me boils down to; if adventuring is common enough to be a profession as opposed to sellsword/mercenary/bounty hunter/bailiff/busybody, the land must be so riddled with problems it probably doesn't deserve to be saved even if it can be.