I'll assume that's a serious question.
The people upset about mail-in ballots are upset about the window of opportunity for fraud which is created when it's possible to count most of the votes on election night, and then have an interval during which someone intent on fraud could, knowing exactly what gap has to be overcome, manufacture votes to do just that. It's thought to be a lot easier than trying to find a few missing boxes of ballots in someone's trunk on election night. If all the votes have to be in by midnight of election day, then it's possible to count the number of ballots even without examining the contents of the ballots and put a hard ceiling on the total ballot count.
It's usually wrapped up with chain-of-custody weaknesses: an in-person ballot collected from and returned to a poll worker at a polling station has a much tighter chain of custody than ballots shotgunned out by mail that can be collected and returned ("harvested") by people other than those to whom the ballots were sent, using means that by definition are outside the control of election officials.
"Well, fraud never almost never happens." Of course, of course, but elections are like ethics: a polity only has free and fair elections as long as there is no appearance of impropriety as well as no actual impropriety; and, it would only be possible to know how much fraud there is if all jurisdictions were energetic about seeking it. What goes on in the US is puzzling, because one team largely favours voting controls that in most cases are unremarkable elsewhere (including Canada), while the other team resists.
Observe everything that goes on in US politics, particularly to capture the big prizes, and then posit a theory that fraud isn't part of the mix. I wish its proponents luck.
www.justice.gov
www.theguardian.com
