I've seen the British election many times, there are hundreds of people in the counting hall for every district. They count, then they shuffle the ballots around and have different people recount. It would take the bribery of thousands upon thousands of people. People who are under constant scrutiny, not just by those around them, but on TV also.
A computer is almost trivially suborned in comparison. Even our most important pieces of security software, such as openssl and others, get hacked from time to time. If such hugely important - massively used, massively tested - open-source projects get hacked, why on earth would you want to risk computer voting?
It's just standard programmer NIH hubris; we think we can do it better, therefore we should.