Signature matching.
There is no signature on the ballot, because we have codified in law the notion of the right to a secret ballot. The signature is only on the outer envelope. The ballot is in a second envelope contained within the outer envelop.
Once the ballot and secrecy envelope is separated from the outer envelope, there is no possible way after that to track a specific ballot and subsequently disqualify it.
This is why the initial signature verification was so important for Republican poll watchers to observe, and why the Democrat-run precincts forcibly removed from the target big cities.