You bring up Star Wars which clearly contains elements of two systems in opposition. The Empire vs the upstart Republic.
Harry Dresden's Chicago is still mostly like "our" Chicago (politically speaking).
Salvatore's Icewind Dale Trilogy features a group of ten towns governed by a democratic council.
China Mieville's New Crobuzon is a republic city-state.
Simmons' Hyperion had Tau Ceti Central—an ecumenopolis that was the capital of the Hegemony of Man.
There's stuff other than (mon|olig)archy out there—albeit rare.
Besides the historical aspect that has already been mentioned, I think a lot of it has to do with the scope of the story. If you're planning on writing a large
series of fantasy/scifi books, it's likely that "Government" is going to need to make an appearance at some point in the fleshing-out of your world/universe. If you have no intentions of including political plots/subplots, then the benign monarchy makes a perfect, stable,
little-forethought-involved background for your story (... and the King decreed).
Whereas a stand-alone fantasy book may be able to get away with not even addressing what form of government is in play. At all. And being that true stand-alone sci-fi/fantasy books are more rare than blue, speckled puppies... well... "it's good to be the King."
