Originally posted by AsianGirlLover:

Yeah, it’s a weird design decision. No troll spawns outside, no troll spawns inside, and sometimes there’s not even any treasure.

Some seeds literally just generate empty caves and it sucks. Immersive, maybe, but disappointing.

It's my headcanon that they were eliminated/looted by Ulf, Astrid, Harald, Frida, Bjarni, or one of the other Vikings we see mentioned on the Runestones.