The point is that humans counting paper ballots by hand in the witness of anyone and everyone is and always will be more credible than any voting machine ever. You can certify the digital chain of trust as much as you want, it will not beat human hands counting paper ballots as anyone and everyone watches.
>you're not thinking about this very hard
Yes, because the commons will not think very hard about a complicated "solution" when a much simpler solution already exists.
>If you can't envision all the ways humans can fail,
Yes, humans fail. It's also not important. Any election worth its salt should be counting multiple times using a variety of counters and witnesses to demonstrate repeatability of the vote.
Again: Humans failing is not important.
What is important is the ability to verify immediately and simply how the vote is being tallied. Machines can and will fail (or more likely be corrupted) like humans, but we can immediately see when the human screws up whereas it's impossible to see when the machine screws up.
It's baffling I'm having to argue this to FOSS people of all peoples, you guys should know better than anyone else that vetting source code and binaries and hardware is a fool's errand for something as important as counting votes.
Nothing beats the brutal simplicity of hand counting paper ballots while everyone watches.