Perhaps it is a complicated business.
I wish I had captured an argument an anon was making a few days ago about how in national emergencies, military lawyers could be put in place to assist civilian courts in navigating matters involving national security complications.
Moving lawyers around is relatively easy. They do specialize, but the core education is the same for all of them.
JAGs tend to stay in the military venue becuse they like it there, it's specialty, understood, etc. Similar to career intertia everybody has.
-- Perhaps it is a complicated business. --
Somewhat, but not beyond an average person's understanding. Just takes time and attention, a little bit of academic/intellectual discipline.