Anger, Self-Loathing, and misguided Hate
Anger is like a fire. They're a useful yet dangerous tool that burns and breaks stuff. When handled correctly, fire can shine light in the dark, give warmth in the cold, cook meals for the hungry, or protect you from wolves. In other words, a well controlled Anger is good at getting work done.
Not everyone has learned how to manage their Anger properly. Some let their Anger go too dim, making it hard to do stuff. Some redirect their Anger at themselves, out of fear of hurting others or believing they deserve it. Some let their Anger spread without a care of who it hurts, as long as it gets the job done. Some learn to concentrate their Anger into a beam of Hate, but don't know who or what to aim the beam at.
Going back to the question "why do people vote against their own interests?" It is Self-Loathing. It is people who are so used to having a piece of themselves set on fire by others that they start setting themselves on fire of their own volition. It is misguided Hate. It is people who know there's a problem and want to fix it, but have been misled about the source of the problems by people who are interested in not getting targeted by Hate.
"Why do they vote to benefit the rich?" We don't have a choice there when either vote would have benefited the rich and powerful anyway. Just choosing between different types of benefits. Money and Power have a tendency to rise upwards, so any aid we give to those struggling at the bottom will end up benefiting those at the top anyway. But I hear ya, giving benefits to the poor and letting it rise away still beats just giving it to the rich and hoping it'll trickle down someday.
I'm not entirely clear if this is a rhetorical question and you wanted to vent for a bit, or if you're genuinely looking for an answer?
If it's to vent, then I kinda understand. It sucks to see someone you love suffer. It sucks again when you think you found an answer and they tell you no. Each time you try again and get your answer rejected, the feeling gets worse and worse until you start blaming them for their own suffering. I've been on both sides of that.
They tell you to stay strong. That love and patience will change their minds eventually. On a level, it makes sense. If a problem has been persisting for decades, it's kinda unfair to think you can solve it overnight. It's likely the solution is also going to take years. But then you spend the time trying only to find more layers to the problem.
If you're looking for some kind of answer, then the best I can offer is it's Trauma. We all go through our own traumatic experiences. Events so bad it takes away a piece of us and leaves us scarred. Those scars change how we each approach or avoid situations. Not many people want to relive their traumatic experiences, even if it would lead to better lives.
Sorry if it's not the answer you're looking for.