They tend to vote against their own interest because ‘one day’ they’ll have more money and what they’re voting for will benefit them ‘then’.
It's interesting how so many people seem to believe voters get a ballot with multiple line items on them, one of them being "Raise taxes on the rich". Or if that's not the root belief behind "people voting against their interest", that economics is the prime motivator for all voters.
Some other theories:
https://mediahub.unl.edu/media/8546
https://www.theguardian.com/society/2012/jun/05/why-working-...