The Mercenaries are willing to sell their services to the highest bidder. They are descended from a collection of military members and law enforcement officers that banded together shortly after the Calamity. Much of their training was passed down through the generations and has since been honed and adapted by near-constant warfare and conflict. Their gear is in the best shape, and they know how to use it.
The demand for skilled mercenaries is the highest it's been in centuries, and these guys trademarked the name, so they must be good.