Because they're not in there to fight major wars, the thing is, the US is not interested in fighting massive wars anymore. Mercenary don't work if you want to make large scale invasions like Afghanistan or Iraq, but they are extremely useful if you want to maintain order via a series of small scale operations in conflicts like the Houthi war or the instability of the ME. The current trend is that the US is pulling out of being the world hegemon, and because of our geography, we don't really need to worry about invasions, so we aren't naturally supposed to have a large military. Ironically, the place that should have large militaries (Europe) doesn't need to because of the US. America being a mostly disarmed nation makes a lot more sense than Germany or Britain being one, and that's a problem they'll have to confront once America finally pulls out of the world.
So yes, Constellis is the real military. It's just that America's military needs have drastically reduced in the last 10 years, and that's a trend that isn't going to stop any time soon.