because the vast majority of the time, we see the aliens in a particular situation. In the same way, we see starfleet officers, and they speak English. Of course, that doesn't mean in China Humans aren't speaking Chinese. And in the same way, just because we see Klingons speaking Klingon, doesn't mean in some country on their homeworld they aren't speaking a diofferent language.
Anyway, how do you know you haven't heard five different Klingon languages? How many different ways have we heard Klingons call for transport? And also, given the universal translator, how do you know that they haven't all been speaking lots of different languages, and they've all just been conveniently translated into English?