Because idiots chose them?
I wouldn't say the world is run by idiots, only run by people who look like idiots when their idealism is challenged by cold, hard reality. The first and last time an American president actually accomplished their goals was... James K Polk. Everyone else probably made it into office and then promptly learned that their goals were not possible without endangering the country, either from itself or from a foreign threat.
Actual decisions are cold, calculated moves by whoever is in power - keep in mind that the leaders of the free world have many advisers. Sure, Bush was an idiot, but to compensate, it's not like all the power rested with him. He had Dick Cheney, he had Condy Rice, he had Rumsfeld, those people held power over him and held power over this country. Obama isn't leaning as heavily on his advisers, but he still uses them - Hilary Clinton is almost always consulted on foreign matters, even things like the bin Laden operation. These people generally know what they're doing, it's just that what they do doesn't always align with what we want them to do.