Why can't Joe Biden's supporters see that he is cognitively impaired?

It seems very clear to me that Joe Biden is in the early stages of vascular dementia, or some other type of dementia. I don't understand why his supporters can't see this. He has great difficulty remembering things, and even putting together a coherent sentence. If he is the nominee, Donald Trump will rip him to shreds. I've had family members who have had dementia, so I know what dementia looks like. If Joe Biden ends up being the nominee, I really don't know who I'm going to vote for. I can't stand Donald Trump, but Joe Biden is most definitely not fit to be president of the United States in his current mental condition.