First, these models are looking at individuals' behavior on average.
Second, if you want to be an infantryman, fly a jet, work with missiles, etc, most of the jobs are military. And there are subcultures found only in the military that some people find very appealing. The models are smart enough to consider such factors.
But the motivations and cultures of the military and civilian employment writ large are VASTLY different. Lumping the civilians in with the military and declaring it all good is way too simple. There are huge chunks of the government where the work is really no different than similar work in the private sector.
The postal service and UPS is only one example -- there are some unique functions and activities in USPS, out in rural areas, but the vast majority of it is pretty much the same as USPS. Wanna bet that postal workers are paid more than USPS workers? Or on their turnover ratios?
It isn't too hard to see the best parts of the government, they're pretty interesting and do important stuff. But those parts make a surprisingly small fraction of the total activity.