Paper ballot boxes are generally supposed to be kept in view of election officials from all interested parties, or locked with separate locks for each party, are they not? Electronic voting machines, by contrast, are more likely to be placed in a private booth with a voter, and would thus be more readily tampered with.
Actually, I don't know why voting machines aren't designed so that the case can be secured with multiple independent padlocks (one per party)? If the machine just has a built-in lock, then anyone with the key must be trusted. By contrast, if a representative of each party puts a padlock on the machine, then unless all parties are crooked, the machine will be tamper-proof.
Do you really believe that a person could go into a voting booth, open up the locked and sealed voting machine, connect a motherboard to the circuit board in the machine, re-lock and reseal the machine, all without being noticed?
I still think it would be much easier to "stuff" a paper ballot box, something that has been going on for centuries. Washington State is notorious for cheating with paper ballots. No matter what system we use, if the people counting the votes are corrupt, the election will be corrupted.