You and I actually mostly agree. Joe Biden could have said, "I understand your concerns, so in the best interest of the nation I say go for it." He could say that knowing full well that there's very little evidence of it and it would take the wind out of the sails of Trump and his base and would kill the enthusiasm for Trump's side. He could have made it appear to republicans that he's being completely transparent even though he doesn't need to be, but didn't. Which is strange.
The concern from the republicans is that votes were continuing after election day, postal workers said they received orders from above to mark late ballots as arriving a day earlier, that sort of thing. There was also the fact that all the final swing states had constant troubles and anomalies compared to the rest of the US. Again, it's probably mostly bullshit, but it sounds convincing enough that you can understand why one side would be suspicious. And if it were happening to your side, you'd probably be suspicious too.