Why does arXiv not have an impact factor? Are there any "un-offocial" estimates of arXiv's impact factor?
arXiv does not have an impact factor because it is not indexed by Clarivate Analytics (for good reason, since not everything on arXiv is sensible).

I'm not aware of any unofficial estimates of arXiv's impact factor, but presumably you can get one by adding up all the citations that are generated in the field (multiply the impact factor of all journals by the number of articles if you have to) and dividing by the number of articles on arXiv.