I guess it is for the same reason that the RED STATES are REPUBLICAN rather than the Socialist Party of America [Democrats]. Say it often and it becomes a FACT, and people honestly believe it is true.
I wonder when the last time was when a politician or the MSM said we are a REPUBLIC? I can NOT remember EVER hearing that term used.
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What else would you expect?
Democrats = Democracy
Republicans = Republic
The MSM suffers the disease of Democrats = Democracy !!!