Voting machines were first introduced because fraud was epidemic in the paper ballot days. Ever hear the expression "stuffing the ballot box"?
Sure, but by now, we’ve found out that the machines are easier to cheat with than the paper ballots. It at least takes time and willing people to fill out paper ballots, so there is a limit to how much cheating you can get done. Not to mention, there is a paper trail so that the cheating might be caught after the fact and nullified.
With the machines, all it takes is one guy pushing a few buttons to rig the vote for each polling place, or, in the worst case scenario, one programmer modifying a few lines of code to rig the vote for an entire county. Once it’s done, there is usually no trace left for investigators to catch.