From a KC-10 perspective:  Because simply being admin gas sucks and is mostly a waste of our time.  Because TACC doesn’t have the tail availability to do a flag and other TRANSCOM-validated things.  Because CENTCOM has too many of our jets.  Because we don’t have a weapons school to speak the same language and properly integrate into a planning team.  Because we don’t have anything to be on the net (yet).  Are they all excuses? Yes. But every time I’ve been involved in a flag, my job has been to sit fat, dumb, and happy in an anchor that is either off the range or outside the engagement area. It’s a self-licking ice cream cone…we don’t get valuable training (except boom fighter contacts) so we don’t value it so we stay admin gas because we don’t have the ability/care to integrate.