Why do cooperatives get better prices from suppliers?

I'm aware that some of the problems of farmers is that they need capital, but quite a lot of them are smaller and don't have it. So they team up, pool their resources and buy seeds, machines, and buy various supplies in bulk to get cheaper prices per unit of supply (like fertilizer)

Now, I can understand pooling resources to buy something like an automated thresher or a milling machine.

They're only going to sell you at that price, you don't have banks that'll lend to you, and it'll take a long time to save up that money. But what causes the prices of supplies to drop when a cooperate bulk buys it for its members compared to poor farmers buying it singly?

(Correct me if I got anything wrong. I got my information from Europe’s Green Revolution and Others Since: the origins of peasant friendly plant breeding)