The answer is that our armed forces are constantly having to compete against other militaries around the world. And if people in our military don't compete at a high level, then there is a high probability that they are going to get themselves and alot of their friends and associates killed.
Outcomes where people die if they don't perform well have a tendency to force people to perform well. Of course, I would be willing to revise my beliefs on School Choice if the same rules applied to the Education Establishment, i.e. if you compete and lose, you die.