The reason Humana (and others) do not allow enrollments by independent agents to be totally online is called "CMS".
Agents are held to much stricter requirements for Medicare Advantage enrollments. If you have seen any of the government hearings on CSPAN, regarding complaints about independent agents selling
MA products, you will understand why.
The telephone portion of the enrollment process includes a voice recording so that the Carrier can prove to CMS (if there is any future misunderstanding) that the client was aware of what they were signing up for and that the terms were clearly explained.
Larry Reader