As I understand it:
1. Because machine-generated content is not as good. Recent technical improvements are (IMHO) showing obvious and significant improvements over last year SOTA tech, indicating that the field is still very green. As long as machine-generated content is distinguishable, as long as there are quirks in there that we easily notice, of course it'll be less preferable.
2. Our innate "our vs foreign" biases. I suspect that until something happens to our brains, we'll always tend to prefer "human" to "non-human", just like we prefer "our" products (for arbitrary definition of "our" that drastically varies across societies, cultures and individuals) to other products because we love mental binary partitioning.