A regular army is supposed to go out, win a conflict ("shoot the bad guys"), go home, get decorated, enjoy pensions and glory; that's the core mission, and its core competency is (or rather should be) to get that done in the shortest possible amount of time (or even not getting engaged in the first place). A mercenary only gets paid as long as there is an ongoing conflict involving a wealthy-enough client. He's not interested in ending the conflict; thinking you can police him in seeing it some other way is just deluded.

An army that doesn't fight is a budget that is claiming to be cut, and you can see that discussion taking place in several european countries. In the United States, however, they have wars to fight, and so the army and its space is a huge industry, with one client, the US government; and if there are no wars to fight, it will quickly collapse.

There are in fact a lot of accusations that the military industry in the United States instigates action for that specific purpose. So they don't only keep alive the fire, as the mercenaries - they also push to start new ones! And I can tell you, the Army has more political force than mercenaries to do this.