It is when you're opponent is too inept to prove it. Not to be mean, but at this point there are only two possibilities: The democrats cheated and the republicans are too dumb to prove what they say is "obvious" to everyone, OR the democrats won legitimately. Personally, I don't think the republicans are stupid. I think they know dang well Biden won, but want to avoid the wrath of the president, and thus are toeing the line.
Or, could be that it's not all that easy to prove election fraud on that scale. It still can be that democrats cheated. It's unknown, but the default assumption that Democrats won legitimately because without proof to the contrary there isn't a reason to assume that they cheated. But I do strongly suspect that democrats would be whining that Republicans cheated if they had the same evidence or less than Republicans have now. And social media would be blocking any claims otherwise. Look how long the Russia/Trump collusion bull**** went on. And some people still believe it happened!