It is an abstraction. As to the burden of 'providers', when one reads about the principles of DID design one finds a deliberate bias toward the identity side (person, thing, etc.): All the subject needs is their DID and to (somehow) verify that it represents their identity. Whatever backend providers inflict upon themselves to deal with that is their problem. This mentality is intended to make coping with identity easy for the 'I' (identity) party.