You're making an all or nothing fallacy in your thinking. These are not reputable mercs on an orderly world, or house troops in garrison. These are pirates, and most of them won't have their own transportation Dropships are much rarer than mechs and combat vehicles. They need to be somewhere that Dropships and Jumpships willing to transport a pirate crew pass through regularly enough to stay operational. Somewhere that has enough people and commerce they can maintain their gear, resupply, and sell off their loot. But it also has to be somewhere with little to no central authority or rule of law otherwise them being pirates is not going to go well for them. Anywhere that anarchic which attracts pirates is dangerous as much as it's an opportunity. Think of Somalia at its worst, then remember these are pirates with battlemechs. The bigger bands will have openly operating fronts in cities but that's not where the majority of their mechs and vehicles are going to be cause cities aren't great for armored forces. Most of their hardware is going to be outside the cities just to give them room to operate. The smaller bands will operate entirely out of the countryside and will do their best to keep their bases hidden though they will obviously send their people into the cities for resources and R&R. The big reason for this is that they are pirates, and they will absolutely attack each other if they believe they have a chance, being pirates they all recognize the potential dangers posed by other pirate bands. So they aren't going to want the place they keep their battlemechs to become known, that makes it a target for every other band of desperate pirates looking for an opportunity.