Since the 1950s, every effort to reduce the size and scope of government has been bulldozed by a political establishment more concerned with furthering its own interests than those of the American people. Between sacred cows and special interest groups, we're always told why nothing can get cut.
With the exception of Social Security, there is no bigger scared cow than the Department of Defense (DOD).
Of course, defense policy is a legitimate function of government—a textbook example of a public good. It's hard, though not impossible, to imagine national security being provided privately. However, it doesn't follow that every dollar spent on defense is effective or even legitimate. It's often the reverse. Sacred-cow status grants relative immunity to the Pentagon's waste and poor strategic spending.
