I suppose because it refers to an amount of something. Other words that do this are also countable, like “amount” or “quantity” for example. So even though it doesn’t really make sense in most contexts to have “two quantities” of something, since you are technically talking about a group of things that are countable, you could theoretically have a second similar group and count it as well. I’m not sure if my answer is based in any grammatical rules, or if that is just my attempt to justify the grammatical use that exists.