TL;DR: I blame FPTP.

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Hm, I’d argue that this is a byproduct of the spoiler effect — I think it’s due to strategic voting. I think that it’s likely not due to people consciously voting against their own interests to benefit the rich (assuming that they indeed do this ­— ie that voting to benefit the rich is against their interests), but instead that the entities that support these sorts of beliefs, also tend to align with other beliefs that are more important to the voters, and “benefiting the rich”, while possibly perceived negatively, is a sacrifice that the voters are willing to make.