I do think this is true (that people mainly vote in their own self-interest). They don't think about it much, even when it affects people they know, even family.
Sadly this is true of my father. He is semi-retired from running his own small business and has a lot of money in the bank (a lot of this from inheritance). He has one grandchild who is a young single parent on benefits (though she is going to college and trying to improve her situation) and his other grandchildren are saddled with massive student loans. Also has another adult child who has needed mental healthcare which is hard to get especially due to NHS cutbacks, and other children who have been on zero-hours contracts and struggling. He has been sympathetic and helpful, and he has certainly helped his family out a lot, financially and in other ways, but doesn't seem to see the connection between his voting habits and what happens to family members.
He is basically a good man, but wouldn't dream of not voting Tory. Has always read the Telegraph and thinks Cameron is a top bloke, apparently. Yes we have all tried to reason with him!