Mainly what the others have already said, there's no financial gain.

The only argument I can see somewhat working is if a company wanted to start the war and simply hired mercenaries to be the guns for it. But even then it doesn't really add up cause business make money by making and selling products, not by having a lot of guns and taking places over. Maybe since corporations can work in the trillions at times they could end up having a corporation war (assuming it could ever be hid from the government), but that's hardly the same as merc's going rogue and taking over things on their own.