Exactly what you said, a while back people were buying things in bulk and then reselling. Sometimes lets say.. those little tiny candy bars, you can buy a bag for 5 bucks, but you sell each for a quarter.. nice profit eh?
there’s another reason too but sorry can’t remember.. I know I am no help.
I think it keeps the retail stores from opening the packages and selling them individually to make more money. For instance: a 4 pack of pudding for 99 cents. If it were legal, they could sell them individually for maybe 50 or 75 cents each making 2-3 bucks on all 4 instead of 99 cents. That’s just a rough estimate, and I think the manufacturers are afraid their business will go down if they give you a choice on how many you buy rather than having to buy the whole pack.