With raids if it can't select a source it defaults to mechanoids since they'll find you no matter what particular hell you've set up in. I thought the order was Roll for bad/good thing-> determine what bad thing-> If bad thing requires a source, select source. I'm not sure if there's a cooldown per faction or if its per type of faction. I think the more pirate/hostile tribal factions exist the more likely is you'll roll a raid from one of them than from mechanoids.

Maybe a shitload of pirates means pawns are generated somewhat differently or you have different chances of seeing old pawns or something like that? More factions fighting each other? If you crank up the number of factions maybe there will be fewer unaligned pawns you run into? Prob it has something going on under the hood where the number of those factions does have some weighting effect or change in how things work that isn't super apparent, but i don't really see it mattering - pirates, savage tribes, mechs, and bugs all seem to only really matter if they're on or off. Because you can't have any diplomatic relationship with the orneriest factions it effectively doesn't matter if you consider all pirate bands to be the same pirate band or not. I usually grab 2 factions for everyone except pirates/savage tribes. If they were somehow grouped into regions or biomes or something it would be more useful to have more factions, imo. The thing about that is that it seems like factions are just randomly sprinkled globally.

Rimworld ancient danger