Why are the people who start the wars never the same people who fight the wars?

A student last year asked me, "Why are the people who start the wars never the same people who fight the wars?" He wanted to know why Bush and the Congress didn't have to go out and fight in Iraq while his brother was on his third tour. It wasn't that this student hated the government or even the War in Iraq. He simply wanted to know why the people in charge don't have to be on the frontlines. What do you think?