The secret ballot and full chain of custody of ballots is a pretty important thing in democracy.
I'm not certain how mail-in ballots are counted currently, but it is certainly possible to have the ballots be in a sleeve, and the ballot itself only be exposed to the machine reading it. Put the ballot 'inserters' under camera if needed. It seems like a simple problem to solve.
Another commenter mentioned that a real issue is that the counting of ballots isn't transparent, which is true. When I vote, all I know is that the ballot scanner increments its 'scanned ballot' count. I can't verify that it actually voted for who I voted for, and they're closed source, so I have to rely on someone else's word that they are.
Maybe this is finally a use for blockchain? lol
Literally all of this is invalidated by a mail in voting system. You cannot be sure the proper person got the ballot, you cannot be sure the person filled it out without it being seen by others or coerced, and you cannot be sure the ballot wasn’t intercepted somewhere along the line and modified. The entire system hinges upon a signature verification process which is automated and obviously needs to avoid false positives more than false negatives, given the risk of disfranchisement. We should expect the signature check to be a poor mechanism to block hijacked ballots, anyone with a bit of sense can see why we should expect it to allow most signatures through.
What we should expect under a mass mail in voting system is fairly obvious, given the means, motive, and opportunities it uniquely affords people in highly contested elections.
It has been interesting to watch smart people (not you) who were warning about these risks years ago be forced to stop agreeing with them because of the political implications that would likely come if we had verified examples of these weaknesses being exploited.
In general, sacrificing this important principle was acceptable if the alternative was to disenfranchise the voter (eg, they are overseas.) Now it seems there are many who want to just throw it away in the name of making voting “more accessible” in general. This is a pretty bad stance imo unless you assume it is cost-free, which it isn’t.