Why is the U.S. military and its budget sacrosanct?

It has much to do with the military industrial complex, which is often mistaken for a conspiracy. In reality, it is a giant mass of self serving interests, made up of thousands of individuals, none of whom has any financial interest in seeing military spending cut; in fact, they want the opposite. So it has little to do with having a military that will be capable of defending the USA against aggressors. I seem to remember that the USA hasn't been invaded for about 200 years.