Because the military is what it is. The environment you find in the military exists for a reason. The reason is to enforce discipline, comradeship and unit cohesion. What happens in a civilian job when things get to nasty, scary, ugly and hard? People quit, right? That can't be allowed to happen in a military unit.
The soldiers that, "just push papers", are still soldiers. Everyone in the Army is a rifleman, that exists moreso in the Marine Corps. Those paper pushers could very easily find themselves locking horns with the enemy and will need to display the same level of discipline as a front line infantry soldier.
People chose to make the sacrifice of their civilian freedoms, to serve in the military, in order to protect the freedoms of the rest of the Americans who either can't, or won't serve.
People in on active duty don't have, nor are they allowed to have civilian jobs. Reserve component soldiers have a civilian job, yet don't have the same post privelages as active duty personel. Free medical care, PX and commisary priviledges, etc.
Most soldiers's salaries are below the poverty level. It wasn't until I made it to pay grade of E-5 that I was above the poverty level, then dipped below the poverty level again when I got married and didn't rise above it until I was promoted to E-6. I'd say that the bennies they get are well deserved.