Musings on Alien Races

I've noticed in Sci-fi that Alien races are often depicted as being completely homogeneous. Same religion, same language, same dress sense, same ideals. This is true unless it is a specific plot point, and even then they are only limited to two warring factions at best.

One notable distinction to this is the Covenant in the 'Halo' series. Admittedly this is an amalgamation of different species held together by a unifying religion, but off the top of my head this is one example where an Alien 'race' isn't treated as homogeneous. There are several belief systems, several cultures, etc. In fact, another game seems to make fun of this sci-fi trope, i.e., in 'Mass Effect 2' where Mordin notes that the human race are good test subjects due to their relative genetic diversity, hence explaining why all other races are so homogeneous.

So, assuming that alien races are not so homogeneous, I wonder whether the reason we have yet to be invaded by aliens is that they have yet to hold a referendum on invasion yet.