Because for decades Republican propaganda has been that Democrats are not just wrong, they're evil Antichrists Communists out to kill all real Americans and must be opposed at all times in all things. And their base bought it to a degree that I think has surprised even the Republicans.
Now it's to the point that if a Democratic politician were to specifically say they were in favor of toilets, many Republican voters would start running around in loaded diapers and saying toilets are a Marxist conspiracy.
These are people who believe Trump is honest and hard-working, and is fighting a deep-state that is behind everything bad ever. They watched him spend his term doing hardly anything but pass tax cuts for the rich and golf while never even giving a thought to the infrastructure or healthcare plans he promised them, and still their faith never once wavered.
And now they take his word for it that the election was rigged by the ghost of Hugo Chavez, Dominion, and literally everyone in the world who doesn't kiss up to him, and never think it strange that an alleged billionaire is begging them for cash all the time.