Because American exceptionalism is a core element of national identity, and includes the idea that all elements of American government are basically democratic.

Because American exceptionalism is a core element of national identity, and includes the idea that all elements of American government are basically democratic. This believe is convenient for those in power who are not very accountable to the electorate and is a deeply held idea by voters in the United States. When deeply held ideas conflict with reality, beliefs will usually stay and reality is the one that is discarded.