>In other words, the 1/2-gallon container, buy two of those, and it costs significantly more than a single 1-gallon container.

Except sometimes the 1/2 gallons will be randomly on sale where you can get like 3 of them for the price of a gallon. Milk economics makes no sense to me. But yeah, it's usually cheaper to buy more than you need and just throw it out if you don't use it, as is the American way.

It also usually sours I think? You can still use that for pancakes or something... not sure because I’m too lazy and throw it away myself.

Inversely, I've also seen promotions where the gallon is heavily featured in the ads, and they're selling the half gallon for full price. Neat, you're paying extra to get less milk!