Paper works, and it works well. It's a system that has worked well enough for thousands of years, and we have figured out most of the issues with it during that time. Anyone that can count can validate a single precinct. You can have one person, or 100 people all standing there watching a ballot box all day for tampering. You can have a whole group of people count the results, or just a few.
In a traditional paper system, swaying a single precinct with "blackhat" methods takes a lot of physical resources, a lot of time, and in most cases a lot of people. Then multiply that by every precinct in the country, and it quickly becomes pretty much impossible to do and get away with. Plus it leaves a physical "paper trail" (in the form of payment for people, communications, and physical materials or the receipts for those materials).
Electronic voting gives us very few benefits, and a significant amount of downsides. And it doesn't matter if it's FOSS, it doesn't matter if it's vetted, it doesn't matter what safeguards are put in place, all it takes is one mistake. One fuckup, and someone can now choose the leader of a nation, and in some cases that leader can change the rules of the next election, meaning it only takes one single mistake to ruin it for many many generations in the future.
And replacing a system where literally everyone can validate a system on voting day if they want to with a system where only a fraction of a fraction of people can even read and understand the code, let alone validate the code (and can't actually validate the hardware, or make sure what is running on the hardware is actually that code, or make sure that the hardware is even what it says it is), and it takes a magnitude more time to do so, just isn't a good idea.