TL;DR: I blame FPTP.

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.

A lot of people have aspirations of themselves being rich and if they can vote like rich people they participate in the rich aesthetic.

• Because propaganda works. If propaganda didn’t work, companies would not advertise products and politicians wouldn’t run campaigns. Rich sponsors fund politicians who promise to look after their interests. Well-funded politicians run better campaigns and win.

• Because politicians are, nearly without exception, above middle class, if not outright rich. They won’t act too radically against their own class interests.

The only solution I know comes from ancient Athens. Sortition -> you hold a lottery to draw representatives. A few extremely stupid people will be drawn into parliament, but idiots are far better than sociopaths, and the current system gives undue representation to sociopaths (willing to climb over bodies if that gets them to power). If one then dislikes the idea of a considerable percentage of bumbling fools (as opposed to cunning predators) in parliament, one must feed everyone well, treat all childhood diseases and educate everyone as well as possible. As if their rational decisions were needed tomorrow.

Often when I see someone accusing people of voting against their own interests, it’s pretty clear that the person making the accusation has not taken the time to understand the values others are basing their choice on.
If I could rob a person and be confident that I would never be caught and punished for doing so, am I acting against my own self interest if I chose not to rob them because it goes against my moral code? No, of course not. But based on the way some people talk about voting against ones self interest, you might think I just cheated myself out of free money. Is it possible that a person might “vote against their own interests” because of a misinformed view? of course, but you’ll never understand a person’s motivations by chosing to paint them with broad strokes based on your prejudices instead of getting to know them individually and trying to understand what it is they truly value.