When I was playing Fallout 3, it was alot easier to know why bandits controlled buildings/supermarkets and the like, because they were trying to suvive in the oppressive environment. However Skyrim is alot richer and hasn't been damaged by radiation, and so I need a reason as to why bandits take refuge in caves/forts and strongholds.
Thanks guys1
They can't live in the cities. They're wanted men. They pick places that are removed from the law, but close enough that they can get to the roads or poorly guarded villages for their raids.
Smaller and weaker gangs are based in caves most likely because that's the best territory they could carve for themselves. Larger gangs take fortresses because they're fortresses. They provide better defense. I think it's kind of a mix of reasons for bandits taking over Nord ruins though. Some might be grave robbing, but others might be taking advantage of superstitious Nords being unlikely to want to dive into what could very well be a Draugr pit, and others might just be desperate enough to flee into a Draugr pit because they don't have any other options.