Speaking as someone who worked as a relay operator for the deaf and hard of hearing, there's likely a reason this doesn't happen. People using these services largely want them to be a transparent layer between them and the thing they're trying to access. Basically the person is there to provide the same access a sighted/hearing person would have. If they form a relationship with the person helping them, they may be less inclined to read sensitive documents. Additionally this gives the helpers a more clear window into the life of another person (which can be used nefariously) rather than, more often, bits and pieces without context.