I don't know about studies but Dr. Barnard seems to be the plant based doctor who discusses this the most. Here's a great talk he did years ago and his book The Cheese Trap.
My personal experience like many others is that cheese is the one thing people "just can't give up!" Whether that's from casomorphines or the fact that it's concentrated fat/calories and salt specifically design to be highly palatable I don't know. I suspect it's all of the above.
It sounds like your parents are the the cheese trap. Milk has a chemical in it to make the calf come back for more. It’s mildly powerful, about 5% the strength of heroin. In cheese, being concentrated milk, it’s a bit more powerful.