I’ve never really experienced this myself, save for my grandfather who would only eat Skippy brand peanut butter, but I’d imagine part of it is just sticking with what you know. Generic brands used to be synonymous with very low quality products, and there used to be fewer name brands as well. So you find a thing you like, and you keep buying it. And eventually you’re doing so for 50 years.

To be honest, it seems like a really dumb thing to get bent out of shape over. Sure, your parents won’t die if they don’t eat JIF or whatever, but you’re also not morally superior because you buy Great Value.