Question

Why do you think items that come in a larger quantity cost less per unit

Answer 1

It makes them have their profit all at once, they are selling more items, and sometimes there's less packaging. The faster they can sell their product the better.

Answer 2

They cost less because there are more items in stock meaning that there isn't a limit here is another way of thinking if there are less Items then they can charge you more per item because they know you are gonna want it but if there are tons of that one item they there kinda like eh then you think oh I can live without that....... Hope this helps