Earlier quoted context omitted.
The problem is that everyone doesn't know this to be false. A substantial portion of the population "knows" it to be true.
The problem is that if you look with an inquisitive eye without having a preference on the outcome there's no way to tell because of the margins in just a few districts, all that is sure is that the Constitution was followed and the valid electors chose the President (which is a major reason why it is designed that way).
Despite what people say, absence of evidence is evidence of absence, if you've gone looking for the evidence and it's something that you should find if the claim is true. There's a bunch of evidence we'd expect to see if there was enough cheating to swing the election, and we don't see it. All we have is evidence of some pathetic attempts by the actual loser to overturn things.