Because we'd have to change the fundamental nature of the military in order to enact significant cuts. In order for us to continue being able to project overwhelming military power rapidly around the globe, we have to have bases all over the world, and those are expensive. We have to have ordinance available to go at a moment's notice, and that's expensive. We have to have personnel ready to go anywhere at any time and that's expensive. If we changed the military to defensive only and allowed the rest of the world to duke it out amongst themselves, we could seriously cut military spending.
So, we have a choice, keep spending big bucks, be the world's police force and enforce a semblance of peace, or cut back and watch the world's enemies kill each other in mass slaughters. Naturally, those who want to cut military spending now would be horrified to watch the disasters unfold and Uncle Sam do nothing.