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.
It is also worth pointing out that while this is labor intensive it can be
- scaled reasonably down. Which allows polling places staffed by fewer people.
- allows a higher number of voting stations as only a low tech physical curtain is needed to ensure privacy.
- throughput is primarily limited by identity verification which takes a cross check of ID document and voting notification card that is mailed to any eligible person once they reach their 18th birthday.
In Germany, we get
(a) a paper ballot
(b) a pen
Works perfectly. And quickly.