It isn't. The US is a military-industrial oligarchy whose elements of democracy exist primarily as theater.
Americans believe they live in a democracy because they've been conditioned through propaganda that their guns make them free. Yet their consent for American imperialism is manufactured by lies from the media, their police steal from them and murder from them with impunity, their intelligence services spy on everything they say and do, and their government is engineered to operate completely divorced from the will of the people. They suffer a quality of life worse in many metrics than other first world countries, with fewer rights and freedoms. But it's cool as long as they can shoot a senator in the head whenever they want.