@Logicfest -- don't know the answer to either question, but here is another one. How much does it cost to bring in those contractors? Could it be that it is actually more cost effective to put some of those groups in charge of certain things than it would be to bring the military in for those functions?
The cost effectiveness might answer your first question. As for the second one, it could be that what contractors can and can't do is spelled out very specifically in their contracts. That's how things work with companies charged with making military equipment, for example. The government specifies exactly what it wants and then private industry submits proposals in hopes that what they offer will be accepted.