Which can be difficult / inconvenient!
Sometimes it's the 10-pak that's cheaper per unit. Sometimes it's the 20-pak. Different products may not be directly comparable. Different varieties of the same product category might appear at two or three places in the same store, not counting end-caps.
I'm guessing this was introduced as a law, in good faith to help consumers make more informed decisions, but I doubt the majority of numerically challenged consumers take advantage of this information.
For example, all pricing labels in-store will have prices like the following, from their weekly ad:
Price per weight only partially solves the problems... lots of products aren't directly comparable in the first place. For example, 100g of product X is not like 100g of product Y. It works well enough with things like raw ingredients (flour, oil, etc) but even once you are talking about things like cereal, jams, mayonnaise, etc. it starts to break down. It also doesn't work well with meat.
Small mom and pop kind of stores don't always have it. But a "proper" grocery store will always list per weight here.