It is some sort of tribalism. Believers can't see it. E.g. we gotta remember that people were gaslighting eachother into pretending Biden is not what could charitably be described as about to be senile.
Refusing to see one self as part of the problem, fundamentally.
They planned poorly with their candidacy; Biden and Harris were the obvious candidates being president and vice-president, respectively, but Biden was too old and they couldn't find a different candidate that wasn't as well known as Harris quick enough.
That said, the Republicans would have the same problem if Trump dropped out or if that bullet didn't miss.
And maybe you’ve forgotten how the RNC rules were changed to support their candidate?