And is there proof that there wasn't fraud? Does the burden of proof fall upon them or the government?
I think you can, to a certain degree. For example: by making the mail-in system more trustworthy, or by eliminating it.
My country (Chile) only uses on-site voting and no one alleges fraud because we know that it would be incredibly hard to steal an election with this system. The system is transparent, votes are counted publicly the same day of the voting.
The electoral service is one of the few public institutions that I deem competent and actually trust in my country, and I think I'm not alone.
To be honest, now politicians are trying to impose mail-in ballot system, but that not the electoral service's fault. That's idiot politicians trying to ruin something that works.
This is a man who, in what any normal person, could admit was a mistake when tweeting about a hurricane having effects in Alabama. Instead he doubled down, tripled down, forced federal employees to make statements that he was right. He even drew sharpie on a map to extend the prediction cone because it didn't show what he said.
There is a clear pattern of behavior here.
Additionally, those who are alleging massive voter fraud in public did not make those statements in court, because it would have actual consequences. Every state certified their results, including those with republican leadership.
So you either have to buy in to extraordinary claims of all kinds of (in many cases provably false) of voter fraud and a massive apparatus (Deep State) working against Donald Trump or accept the much more obvious and plain result.
If you listen to the call with Georgia, when the Secretary of State pointed out that they had investigated several of the claims, Trump's response was that the investigators were either incompetent or actively working against him.
It goes beyond the pale.
I mean... why is it that if someone tells me that Sun rises in the west virtually everyone just ignores this as an obvious nonsense, but when someone says... a more nuanced but nonetheless proven-false controversy (I'd intentionally refrain from any examples - even the ones that were already provided - to avoid even a possibility debating any of those), some don't outright dismiss it.
Maybe I'm a weirdo, and maybe it's simply not possible with humans - but I still hope for a society where (argh, okay, let's invoke Godwin's Law) the next Hitler won't happen not because of some speech laws but because simply no one would listen to the lunacies seriously.