The process you described would require a group of humans to conspire together and carry out voter fraud. Which is not only illegal (and thus carries high risk on an individual level), but probably difficult to pull off without anyone finding out about it. Compare that to totally opaque code running on a system only a very few select people will ever get to observe. Not only that, the average person may not even be able to spot malicious code even if they were given the opportunity to stare right at it. It would be pretty hard not to notice someone dumping ballots or stuffing ballot boxes if you were observing a counting room, even with no prior knowledge of the process.