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 as to the practical nature of her actions because she can see them play out in her community. Th individualist operates solo; everything for him is about your vote your candidate. This precipitates in a divorce between the individualist and the material outcomes of his actions. This gap, this absence of practicality we could call it, leaves a vaccum where symbolism can enter—this becomes a problem not when symbolism is simply encountered by thr individualist, but when the symbol becomes the act, the vote becomes a kind of personal expression, and any thought for collective consequences fall 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 the collective action is “good.” For example, concepts of individualism are fundamental to bearing forward the human rights to consent and bodily autonomy.

However, the setting and background of your question is the USA, a country that has 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 into the heart of US 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 the intermingling of genuine compassion 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 degreesy varrying between people, the vote loses its material value and becomes nothing more than a symbolic moral statement.

This doesn’t mean that 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 “cuz 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.