US government military is so much better. Skill fade is brutal. The US military spends a fortune training and exercising and refreshing and retraining, over and over. Mercenary companies simply don't have the funding or facilities to do the same.

Typically, the employees are contractors. Some of them might still be in date for some of their professional qualifications, some of them might be years or even decades from their last professional military refresher. I'm sure they can get by swapping tips and visiting gunranges, but it can't compare to spending nine months of the year on exercise or deployment.