Well.. I try not to insult people here (they might actually be real Americans) but sometimes it is difficult.... especially when I get insulted on more or less a daily basis.
But I think on some level they simply cannot believe it. They see Trump as some kind of hero/best president/patriot who ever lived. And they cannot believe a large enough group of the population is in total disagreement with them about that....And their guy, Trump, can't or won't believe that he lost, so he goes around telling them and everyone else that the dems stole it. ( although I did hear him say recently that he lost... But he still followed it up with something about it being "rigged")