Why, if speech is simply a reaction to statistics we are tracking and behaviours that have been rewarded, would the first utterances have been made?
That criticism can be lobbed at all abilities that we claim came about due to evolution - which, to be clear, is all of them. The statistical model would be the mechanism, but it wouldn't be the reason why it evolved. That answer is relatively boring, and is the same one as all evolutionary processes: it appeared randomly from mutation, and it provided benefit to those that had it.