Sure there are potential exploits against paper
Auditing techniques can sniff out ballot stuffing attacks, but even if you assume they can't or that the auditers can be corrupted...
The important thing is that ballot stuffing requires one or more people with physical access to the ballots, in every single precinct where you want to "stuff". You can't hack paper from halfway around the world, or compromise 10,000 precincts at the same time. Ballot stuffing doesn't scale the way hacking does, and you only have to get caught once and have someone spill the beans for the whole scheme to unravel.
Plus the US has a really great defense against ballot stuffing even if you assume every person in the government of an entire state is corrupt and makes laws against audits - the electoral college. If one party has such control over a state that they could successfully attempt this, that party would win the delegates for that state anyway so there's nothing to be gained at least not on a national level.
Biting the hand that feeds IT
