I'm not one of the withhold-voting, or vote 3rd party people, but I think it's probably driven by deep moral disgust of both parties preventing people from being able to willingly vote for Hitler 1 over Hitler 2. As an extreme example (which probably is the case for some Americans, but not "significant electorally"), if I was a Palestinian-American, and had many family members killed by weapons supplied under Biden's orders, I probably wouldn't be able to bring myself to vote for his number-2, even if I thought it was the lesser-evil. People have different levels of emotional empathy, and different principles and philosophies. Refusing to participate in an unjust system is a valid stance.