Because open source software doesn't make electronic voting 100% foolproof either. It's hard to prove the software running on the systems is the same as the sou

Because open source software doesn't make electronic voting 100% foolproof either. It's hard to prove the software running on the systems is the same as the source code you've read. And you still have other issues like radio waves being emitted based on the computers' calculations, which (in theory) makes it possible to determine what you've voted. Researchers in The Netherlands proved that it was possible with the machines there, which is why we went back to pencil and paper.