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-
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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.
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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?
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#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.
The real Joe Biden has not been seen since before the 2020 election. I don't know what happened to him, but it is clearly not him on stage. Your ear lobes don't magically attach to your face, you don't suddenly grow a mole in the space of days, the bags under your eyes don't just disappear when you get older, they get bigger. There are videos that also appear to show a rubber or silicon mask, stretching his lower lip or causing unnatural wrinkles on the back side of his neck. Not sure who the guy / guys in the mask are.
The Joe we see is one of 4 body doubles. I believe 2/4 got incapacitated. People get old, have cognitive decline. Same goes for body doubles.
There are also videos of Joe walking thru a parking lot without any protection, and getting on a vehicle while carrying his own bags, appearing to be unescorted. Certainly no Jill around.
Trump has 2 body doubles that we've seen before. While Trump is famous for the maga hat, I don't believe Trump likes wearing hats, and the guy you see wearing a hat is a bit chubbier than Trump. His hair is different too, maybe the reason he wears the hat. He is one of the doubles. Often appears at rallies and public events of lesser consequence. The real Trump is the one you see in tv interviews, where he has to think on his feet. The real Trumps skin around his eyes are lighter.
The Biden double in the debate is the closest non mask version of Biden they have and he is suffering greatly, but I think they feared a mask might be detected on camera by people with 4k screens and hi def tech. Therefore they had to roll out dementia Joe (double), pumped him full of drugs, but it wasn't enough. His neurons just aren't firing like they used to.
phew, that would suck, all they have left is one double and he's falling apart.
Jim Carrey is too spry, then, no-one would believe it was him.