Voter fraud is a serious crime, and unlike other similar crimes (like a fake credit application), the perpetrator doesn’t stand to make $$$$ for each act. They need to commit the crime at scale to change the result.

If this form of fraud were happening en masse, surely there’d be tons of evidence of it?

If campaign workers were going around trying to bribe people to vote, some of the people they are trying to bribe might unexpectedly be supporters of the other candidate, and would have a strong incentive to collect evidence of the bribery attempt and report it. It would make for some very interesting hidden camera videos and would be explosive news in the modern political environment.