There are 3 volunteers to tell, write and cross-check the paper ballot; and it's a public audience, meaning that there are a bunch of witnesses, including families who want to teach kids why the votes can be trusted, and party representatives who want to check that the election is not tampered with. It's hard to cheat when so many people can testify.
Getting a few hundred people to do anything without it leaking is hard. Plus, the paper records are retained, so a recount could specifically identify the culprits.
It's not easy, candidates have the right to appoint agents to verify the voting process, the ballot counts, the correctness of the markings and to certify the count and confirm the ballots are properly sealed in case of a judicial recount.