If Canada and the US merged, would it be twice as hard to vote? Of course not. Voting is the most trivial govt activity to scale up! It is the equivalent of an embarrassingly parallel pb.
The "300M citizens" excuse is even less valid in the context of presidential elections, because each state plans it independently from the others. So it is more like 52 tiny countries voting together!
It is parallel, but not all jobs are run on the same hardware/software and you can't re-run jobs which error out. In fact, some jobs don't error out but should be invalid and if discovered as such might cast doubt on the authenticity of other jobs, etc. etc. So it's really not as trivial as you make it out to be, and neither is anything else which involves coordinating 100M+ people doing a particular task.
The "300M citizens" excuse is even less valid in the context of presidential elections, because each state plans it independently from the others. So it is more like 52 tiny countries voting together!