Why does arXiv not have an impact factor? Are there any "un-offocial" estimates of arXiv's impact factor? I am interested to know where arXiv stands against the journals. Not being peer-reviewed, and being so huge, certainly could contribute to arXiv having a very low impact factor, but since ultra-high-quality work also gets published there, it might still have a higher impact factor than some journals that only contain rather poor quality work that couldn't get published anywhere else. 

Of course the "whole of arXiv" could have a different impact factor than specific arXiv channels, such as "astro-ph" or "quant-ph".
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.