D&D 5E adventurers in your world: common or rare?

[Fanaelialae]

What are adventurers if not raiders who target only monster villages and lairs?

At least IMC, lots of things.

My PCs frequently explore (for the sake of discovery rather than treasure), negotiate, defend, and subvert. Sure, they sometimes attack monster lairs, but killing the monsters and taking their stuff is at best a secondary or tertiary objective. There have been quite a few monster lairs that ended in words rather than blood (and no, it wasn't the unconditional surrender of the monsters, but rather that the PCs reached an peaceful understanding with them). In one instance the PCs' primary objective was assimilation (they chose to make a party that was completely made up of the followers of a proto-deity, so they sought to convert nearly every sentient creature they met). So, yeah, lots of things that would be uncharacteristic of viking raiders.