Because I have never seen a coherent definition of what "free will" is supposed to mean, let alone how to empirically test for its existence. What is this "will" free from? The workings of the brain? If that is the case, then we are talking about the supernatural. If not, then we know that the brain develops according to genetic instructions and the environment, and we also know that both these things can go wrong by no fault of the person.
I suggest reading http://plato.stanford.edu/entries/freewill for a pretty good overview of the arguments in favor of the existence of "free will" (which itself has many definitions).
Which isn't to say you're wrong, but I think it's pretty uncharitable to say there aren't any "coherent" arguments in favor of free will. You did say you haven't "seen" any arguments, which I suppose is not the same as a claim that are none, but a cursory glance at that article should be illuminating for you. People have been thinking about this for a very long time, and no consensus seems to have emerged. :-)
