> I've literally never even seen a recipe for anything that says "Add one box of brownie mix".

You don’t see that recipe because the only place most people see it is on the back of the box of brownie mix.

My family has predominantly made boxed mixes my whole life (though I think my grandma often made cakes from scratch). However, I haven’t seen people in my family use cake mixes in other recipes other than what is on the box.

The one exception might be a cookie recipe my grandma had that used jell-o mix, I think. But it also may have been generic gelatin, as they were chocolate chip cookies, there was no fruity jell-o flavor at all.

You’re right that people like what they’re used to. If you’ve only ever had cake from scratch, it’s going to be good, it’s still cake. The ones I’ve had, they are a little more dense and dry, while the boxed mixed have tended to be more moist and airy.