They have to keep him from the public to hide his mental decline. One of the saddest things to witness is when a person loses everything they ever knew, even themselves.
One main aspect of a degenerative condition is that it doesn’t get better. Hiding Biden will not allow him to regain his strength or mental capacity. It’s actually the opposite. The more he is hidden, the more apparent it will be that he is slipping once he returns to the public eye.