It's in the movies, if not directly addressed. The imperial command structure is exclusively human, as are their footsoldiers, and in A New Hope Luke sees joining onto this as a more desirable career path than anything to do with non-human cartels or businesses. It also seems like at least some of its foundational support comes from humans that had directly been oppressed or threatened by non-human institutions, between Naboo and its invasion by a non-human federation being a rallying point for Palpatine's bloc and Anakin having literally been enslaved in a society that seemed to put humans on the bottom.
Like this is clearly part of the worldbuilding, especially given that per Lucas the Empire is a mashup of America and Nazi Germany with the human dominance angle being a clear analogue to the white supremacism of its inspirations, it just doesn't do a good job of addressing or articulating it.
It really is. Lucas had a few good points, and maybe some writers have done something interesting with the setting, but it's ultimately just vibes based slop and even Lucas didn't do a very good job of articulating his points.
I think the most impressive thing about the franchise is that a janky sci-fantasy samurai cowboy movie about Space-WWII-Vietnam-War-mashup in space managed to mainstream and legitimize sci-fi as a genre.