Because he is a human being too, and has a right to feel secure in knowing who he is interacting with. If something happens, he can later describe the person involved, for example. Police will ask for descriptions, police artists will sketch portraits from the description, etc. The whole point of the niqab is to hide individual identity.

People interact in a limited way over the phone. A person can claim to be someone, but when it comes to crucial issues, no one will just take their word for it over the phone. They will insist on verifying their identity. For some purposes, only a face-to-face meeting will do.

Amazing  that this now has to be spelled out, precisely because some are normalising an extreme practice meant by men to prevent women from interacting normally in society.