Humans are, of course, the most ubiquitous species in the galaxy. Which by default makes them the worst troublemakers in the galaxy. Gav Daragon. Ulic Qel-Droma. Exar Kun. Darth Bane. Lord Kaan. Iaco Stark. Palpatine. Anakin Skywalker. Sate Pestage. Ysanne Isard. Even, Luke Skywalker, to a limited extent gave the New Republic that snot-nosed Jedi Kyp Durron. Wouldn't the galaxy be more peaceful without the shenanigans of these Humans?

Coruscant, Corellia, Alderaan and Naboo has a large Human population. The shipwright nobles of Kuat are Human. The Imperial forces appear to be exclusively Human. A non-Human Senate Guard is yet to be seen. Why this insidious proliferation of a single species thoughout the galaxy?

Well, because we, the audience, would relate better to the story... and because it'd cost more to fit masks (latex or CGI) onto all those hundreds of actors and extras! :)