You could also determine that person X voted for an unpopular politician and ruin their life online. The reason why voting is anonymous is the realization that an election should not degenerate entirely into a popularity contest driven by peer pressure.
If you want to audit mailed out ballots, use two envelopes instead. The outer one will carry a unique id, the inner one will be completely anonymous and contain the actual ballot. All the auditing can be done on the outer, unopened envelopes. Then they are stripped off, all the inner envelopes are thrown on a heap and only then opened and counted.
Of course, even then, the paper ballots contain traces of the voter's DNA, so the truly paranoid will not be happy either.
No, it's so people can't sell their votes, or be told by their parents/employer/local gangs that if they don't verify they voted correctly, they will be hurt. It's not cancel culture or popularity contests or whatever. Voting is definitely supposed to be a popularity contest based on peer pressure.
At least for where I am in the US, that's basically how my mail-in ballot worked for last fall's election.
The completed ballot goes into an opaque sleeve devoid of identifying information, which then goes inside of an envelope. The envelope has an extra "flap" where you put your information and signature, and then it seals onto the envelope.
When processed, the flaps have perforations that can be physically separated from the rest of the envelope and validated and retained for audit purposes, and later on the collection of sealed envelopes can be opened and the ballots counted without having any attached personal information.
If you want to audit mailed out ballots, use two envelopes instead. The outer one will carry a unique id, the inner one will be completely anonymous and contain the actual ballot. All the auditing can be done on the outer, unopened envelopes. Then they are stripped off, all the inner envelopes are thrown on a heap and only then opened and counted.
Of course, even then, the paper ballots contain traces of the voter's DNA, so the truly paranoid will not be happy either.