It's a lot harder to organize multiple staff at multiple polling stations in multiple precincts - enough so that it's unlikely someone not in on the plan won't

It's a lot harder to organize multiple staff at multiple polling stations in multiple precincts - enough so that it's unlikely someone not in on the plan won't notice what's going on, all of whom are willing to cover for each other and who won't leak, and who can somehow protect the ballots against independent auditing, to conspire to manipulate paper ballots than some guy in another country just compromising the polling station and tweaking the numbers slightly to give one side an edge. Election fraud is possible in any system, but it's a lot easier when the system is purely computer-based.