The state of my washing machine doesn't have to change if we just tax billionaires.

That's the heart of the issue, though, isn't it? Most people do care about the state of their washing machines even as countless children have nothing to eat. People chastise their kids for not eating their vegetables by saying "kids are starving in Africa," without doing anything to help any kids in Africa. People want more for themselves even while acknowledging that others have so much less. Studies like this assume that human selfishness is negligible, while it's actually one of the largest variables that needs to be factored in. Most people don't actually care about human suffering unless it's happening to someone they personally know - they care much more about their washing machine.