Most of our decisions are based entirely subjectively from our own experiences, since economics and most social sciences lack an objective standard. Thus, the "veil of ignorance" would either result in our "agreement" either being utterly without standing in reality, or we wouldn't come to one.
Furthermore, even assuming we came to a conclusion that wasn't based on false premises (as any agreement we came to under the circumstances would be), we wouldn't have any right to impose it on those who are wrong, for the reason that individuals have the right to make mistakes (assuming we are unequivocally correct in our view, ignoring the impossibility of that occurring)
If our decisions are based entirely subjectively from our own experiences, since economics and most social sciences lack an objective standard how would we know if something coincides with reality or not?