What’s different about in-person voting? You show up, you vote, you leave. Are they going to take fingerprints and DNA samples of the polling center?
In theory, a bad signature at in-person voting could be cause for arrest, and we could actually do it.
With the mail-in voting, the person isn't present. You can't possibly find the person. No matter how obvious the fraud, there is nothing to be done about it. The person is certain to avoid arrest. No charges will ever be filed, and some people will then conclude that fraud doesn't happen.
In the one case, the criminal faces a small risk that is easy to dismiss. In the other case, there is no risk at all.
There's no police officer hanging out at the polling location to arrest someone for signing their name incorrectly on the voter roll. At least in California, signatures on the rolls are only verified after the fact. At least with mail in ballots you have the recourse of not counting the vote if the signature does not match. For the in-person roll you can't even do that!
Not meant to sway the argument either way, simply an anecdote - in Pennsylvania when you vote you sign your name next to the signature on file. I've seen people be given a hard time when the signatures are wildly different.
With the mail-in voting, the person isn't present. You can't possibly find the person. No matter how obvious the fraud, there is nothing to be done about it. The person is certain to avoid arrest. No charges will ever be filed, and some people will then conclude that fraud doesn't happen.
In the one case, the criminal faces a small risk that is easy to dismiss. In the other case, there is no risk at all.