I think you make a good point. Purely from the public trust perspective, paper and pencil seems so simple it makes people trust the system is secure. If the claim is it's cryptographically impossible for this system to be cheated, most of us would still have to trust the experts making those claims.

However, it's beyond simply the public needing simple systems; the number of experts that would be required to lie about the certainty of a system is low enough that you really do create a less secure system. Paper ballots are harder to cheat because the systems scale in a way that you require more people in on the heist, which is the hardest part of any scam.