Aside from the obvious "he has dimentia" answer.

Why was it so bad?

Many many people believe there are body doubles, actors in masks, and CGI that are all filling in for Joe.

So WHY couldn't they have sent out an actor in a mask and had him at least be coherent?

Thoughts-

1. there are no doubles. It's just all Joe all the time, he's doing his best, the dems screwed up and Jill is making him stay in it till the bitter end.

2. the doubles, actors etc are controlled by the good guys and the feebleness must be portrayed as-is because a) humiliate and punish the dems b) the people have to see c) other reasons?

3. #1 or even #2 but intentional the other way because "they" hate us and want to rub it in our faces that "the"y are in control, they don't even need a good candidate, they don't care, we deserve Biden's trouser chili that's all we're worth. The destruction of the USA on full display for all the world to see.