I have given this some thought too, and my thoughts were that it was either a degeneration/evolution from a previous state where they had a more conventional language (like the "how do you get one one-hundredth of an eye?" discussion in evolution), or that metaphor was the form of communication you used in formal statements, or when you wanted to be perfectly clear about your intentions, which one might want to do with aliens. So just like some languages have different ways of speaking depending on the speaker's gender or social status or what have you, maybe theirs has a formal case for diplomatic encounters or something.