It's a UX tradeoff. People are used to services having unique usernames for everyone which they can use to look friends up. No one wants to say oh just add me on X I'm Qmeufhj32f84hfnejcn438f. Or Add me on X I'm one of the 100 people with real name Bob. Long term it doesn't need to be physically centralized and would make a good candidate for a blockchain with consensus. It's already an append only merkle-tree which is mirrored in other nodes for privacy of public key look ups.
The other effect it has is it allows us to have a global filesystem with all the semantics people are used to from filesystems with human readable paths of the form /$username/$path_in_users_space
The other effect it has is it allows us to have a global filesystem with all the semantics people are used to from filesystems with human readable paths of the form /$username/$path_in_users_space