Ignoring some of the stupid comments above ghouls and what not above, I have also wondered this myself. Essentially, our main character can also be considered a raider in it's own way. You visit places, you find new places, you kill the hostiles and then you loot the crap out of it. But I think the general idea is that Raiders are supposed to be your standard enemies in order to simply the fulfill the requirements of having enough low- level enemies and what not. However, I also don't see a reason why joining a Raider group is impossible. They do recruit, in fact you can often eavesdrop on their conversations and read terminals how recruits came flocking to them and increasing their numbers for whatever personal reasons they have.
Now in gameplay perspective, I imagine raiders quests would be your generic kill/occupy type of quests so maybe they're afraid of repetition? Although, considering there are a large number of raider gangs with specific names I don't see why you wouldn't be able to join one group and then essentially play the game of gang turf war and what not. You know, having your own territory, being able to spread your influence and conquer hostile gangs terrorities and what not. It could essentially work like a extended settlement mod, with more recruits joining as your fame grows and what not.
Throw in some well-developed special NPC's and it could work. But I do think that, with the addition of settlements raider factions become obsolete.