A why-question such as the one in the title can be interpreted in at least two ways. On the
one hand it can be interpreted as asking for evidence that supports the assertion that there are no directionality parameters. Another interpretation, taking it for granted that its true that there are no directionality parameters, asks why the language faculty should be put together in that fashion.
I will touch on some evidence of the standard sort in the first part of this paper (introduction and sections 2 and 3). Subsequently, in section 4, I will move on to the second interpretation of the why-question.