It's kinda sad really.
They want to cover for him. Until there's no him left.
He is not far out of full blown dementia.
A simple investigation into a potentially compromising position held by Hunter kicked a bigger nest of hornets that major political families were involved in, and keeping Joe safe became paramount, because if he went down while he was compos mentis, theyd all go down.
That's when they decided that he was going to be their nominee.
The real presidential pick was to be the VP.
They just need to keep him going long enough, and pull out all the stops on the ride to crazy town to have him pull out at the last minute and their dear dark goddess coasts across the line to make it all go away.