Lots of people have already given good answers about the controls in place, rational individual's risk/benefit calculations and the statistical visibility of large scale fraud.
Add to it that, in a normally-functioning democracy, the party benefiting from the fraud (i.e. those opposing the spoiled ballots) has every incentive to keep a lid on this type of clumsy, easily detectable, behavior:
At small scale, it doesn't "move the needle" and won't significantly alter an election. However it risks reputational damage if found out.
At large scale, it would require coordination across many different cheaters. That has a much higher risk of being leaked out and a party caught doing that would, hopefully, be shunned by voters.
There are, at least in the US, other safer ways to influence elections, such as gerrymandering and manipulating voting regulations.
And... you can always claim that the other party is doing those things. No proof needed.