What stops the military in developed countries from taking over the country?

This happens a lot in Africa and elsewhere, where the military suddenly decides to overthrow a political leader, democratically elected or not. But this never happens, as far as I know, in developed countries, like Canada, Sweden, the United States, etc. What is different about developing and somewhat developed countries that prevents the military from taking control of the country that has so often happened to the least developed countries like Chad, Niger, Sudan, etc.? What stops the American military from surrounding the White House and overthrowing the President, since this very thing happens in impoverished countries all the time?