Sure they can be. But as shown by every single WotC adventure those are made with the foe-creation rules, not with the player-facing character building rules.
We know for a fact that PC classes are not an in-world occurrence. Because there are named NPCs in the monster books that don't follow the PC rules but have the same name. First example is Druid in the MM, 4th level casting and no wildshape. The words exist, you can call someone a fighter just as easily as calling them a soldier or a savage, but the classes themselves are not in-game constructs. So another adventuring party has no need to us PC-facing rules. They do as every NPC every published by WotC for 5e does, use the monster creation rules and give them some iconic abilities.