It's not an accident. Nation-states spent an awful lot of time and money building up professional armies precisely because they had long experience depending on mercenaries. They knew that there were only two ways depending on mercenaries ever ended: either the country couldn't pay the mercenaries enough, in which case they sold them out to a higher bidder; or it could, in which case the mercenaries would eventually get tired of only getting part of the country's wealth, so they'd overthrow the government and take all of it. Professional armies let the nation-state escape this trap.

Unfortunately professional armies fail the one test that matters these days, which is the number of opportunities they provide for private actors to siphon money into their own pockets. So we have decided to ignore the lessons of history, because private profit today is worth social collapse tomorrow.