You understand paper and pen. It's right there. A child fully understands the dynamic.
As soon as the process moves into a machine, you cannot even tell if your checkmark is really counted.
This is doubly worked into distrust, by the amount of bugs, weird isses, and strange behaviour software exhibits to end users. People are trained from youth, via personal experience, how incredibly buggy and untrustworthy computers are.
They crash, can lose data, be wrong, and the worst of it? The layman cannot even figure out why. Computers are a black box, a mystery, they function by magic.
No expert, no claims of audits, or oversight, will ever, ever refute what end users know. Computers make mistakes all the time.
Taking these impressions and incorporating them into the votng system is madness for democracy.