[mikeash]

Hack voting machines: trick or bribe an election worker to plug a USB drive into all the machines a week before the election.

Hack paper ballots: bribe the multiple people, including representatives from each party who cares to send one, to look the other way while you steal a box full of paper ballots and substitute your own.

It’s not impossible, but it’s much harder to mess with paper ballots at scale and much easier to secure them.