Good question. A voter registration card can be checked against a signed envelope. But you can't check the ballot because that is not signed- it's supposed to be secret.
And because they used the same tactics to bully an election committee- and it worked, for a time. Welcome to the democrat socialist party/
That battle would have had to be mounted in every state. State's control their own elections within certain parameters. Covid, covid, covid.
Hopefully, the outcome is people vote at the polls with few exceptions, voter ID, simple counting machines with open source code, rigorous auditing of elections. Absolutely no internet and no last minute updates.