I think most aliens are humanoid because way back in the day when the first movie was made it was helluva lot easier to make. And then they just ran with it. XD
And the racist thing is kind of a part of the, I dunno, "message" aint it? KOTOR for one was all over how cruel and judgmental humans are. It's like they came in, took over, and decided all the other species were only useful as slaves.