Late but here’s my model of the situation. Sort of a WIP and very new but a /gen effortpost, so I welcome thoughts:

It’s individualism versus collectivism. The collectivist understands intimately the function of working together for the protection and future of the group. There is no doubt in her mind about the practical nature of her actions because she can see them play out in her community. The individualist, by contrast, operates solo; everything for him is about your vote, your candidate. This leads to a divide between the individualist and the material outcomes of his actions. This gap—this absence of practicality, we might call it—leaves a vacuum where symbolism can enter. This becomes a problem not when symbolism is simply encountered by the individualist, but when the symbol becomes the act, when the vote becomes a kind of personal expression, and any thought for collective consequences falls by the wayside.

“Ordinarily,” if we imagine such a thing exists, these two identities intermix and act in a complex and altogether non-problematic way; I don’t wish to imply that individualism is simply “bad” while collective action is “good.” For example, concepts of individualism are fundamental to advancing human rights to consent and bodily autonomy.

However, the setting and background of your question is the USA, a country with deep, deep historical ties to white supremacist, capitalist, colonialist, even fascist values, all of which hold the individual as intrinsic over the collective. The result is that hyperindividualism is catastrophically rooted in the heart of U.S. society—even in progressive and leftist spaces!

So, when you see a pro-Palestinian proclaim abstention or that they voted third party, you are witnessing the complex outcome of genuine compassion intermingled with the values instilled by white supremacy and individualism. And so you hear the phrase, “I just can’t in good conscience vote for XYZ.” To degrees varying between people, the vote loses its material value and becomes nothing more than a symbolic moral statement.

This doesn’t mean the leftist non-voter is a white supremacist, of course! Rather, it’s that they have been deeply affected by the presence of those values in their cultural context and have not yet had the opportunity or experience with group frameworks to question their assumptions and reassert the significant importance of collectivism.

So, in conclusion, the unnuanced TLDR is “because America is a racist capitalist hellhole.” The good news I conclude from this, though, is that collectivism can be learned and promoted. Cultural values are definitely not static, and perhaps with education, support, and time, mindsets among leftists can be shifted to better support the whole of the community.