I'm just gonna throw the obvious out there. Because military airspace NEEDS to be managed by the military.
That is correct. And equally, military operational aircraft need to be managed by the military, for instance fighter controllers, helicopter controllers, ship controlled approaches.
Also, for many military pilots, procedural IFR is not the day job. When they need ATC they need it military style - vectoring and GCAs.
Of course you could merge civvy and mil ATC but you will either end up with the same thing in all but name, or loss of operational capability. There is not a better way only a cheaper way.