I feel like "because profit" isn't what you're looking for, but I think that's pretty much the answer.
As much as necessities are, for the most part, necessary, the way they are produced, marketed, and distributed doesn't differentiate them at all from, say, candy bars. You can buy Charmin and Snickers, or you can buy the house brand. They're both products being sold. :/
I think the quantities per package deal is similar across products, too.
Despite the price savings, I don't always buy bulk, simply because I don't want to be a warehouse (I live in a small place with a cat who likes to chew on fresh toilet paper rolls. Thanks, cat.), but you're dead right; you can make some smart calls and save some bucks!