Nick Nicholas’ answer to Why are there so many languages in the world?
Firstly, because we are not even sure that there was monogenesis of language. That is, we are not sure whether language originated in a single contiguous community of humans, or multiple communities.
Myself, I suspect there was monogenesis, but that’s a hunch; and the serious work on the origins of language is subsequent to my training as a linguist, so I’m not across it.
But even if there were a single origin of language, any trace of it is long since effaced, under the waves of language change after language change, millennium after millennium. In our mental and scholarly modelling of how languages have come to be, the single mother language doesn’t make any difference: there could have been one, there could have been a hundred, we’ll never know. So it doesn’t matter to me.