The prior probability of voter fraud is the number of elections in which actors have engaged in fraud / the total number of elections that taken place. Consider any of the countless elections that have taken place around the world and throughout history. Do you believe that the number of them in which actors engaged in fraud is so low as to be incredible, when you think about countries and time periods that are not the country and time period you presently live in? Peruvian elections in 1960. Scottish elections in 1990. Zimbabwe in 1994. I'm just naming random countries/times here. What percentage of these do you think involved fraud? That's what the prior probability is.
Why?
Frankly, I would be glad if we would actually ban electronic voting machines, and use paper ballots. There were some fishy things happening in states[1] that didn't get as much attention. It's much harder to commit fraud on larger scale with paper ballots, since everyone knows how they work and it is much harder to hide things.
[1] https://www.dcreport.org/2020/12/19/mitch-mcconnells-re-elec...