That's the whole point of "burden of proof". It means if you can't prove your claims, there's no reason anyone should take them seriously. I could claim there's a city of aliens on the dark side of the moon. According to you, the absence of proof doesn't mean it doesn't exist. But why should anyone believe me that it does?Sufficient admissable evidence to win election fraud cases in court is hard to come by, but that does not mean that the 2000 Mules movie was wrong in it's conclusions. The burden of proof lies with the accusers of fraud, but the absence of sufficient evidence to prove fraud does not mean no fraud existed. Nor does it mean the election results were therefore accurate.
Ultimately, this is just a matter of faith. Trump supporters want to believe the election was stolen, so they will. And they'll cling to anything or anyone who validates their beliefs, regardless of evidence.