They were only loyal to their commanders. And as you no doubt remember from the historical examples of Caesar, Pompey, Marius, contemporary Afghanistan, this is not a recipe for domestic bliss. So here was Rome, stuck with a bunch of expensive and bloody wars against Germanic peoples who were really good at fighting, and then they had a great idea. Why not fight with these guys? So they essentially hired them, and soon the Roman legions were teaming with these mercenaries who were loyal mostly to gold, secondarily to their commanders, and not at all to Rome, which was a place that very few of them ever even saw. I mean, why would they even give a crap about the health and well-being of the Roman empire? Am I allowed to say crap, Stan?