The purpose of a dictionary is not to define individual words. In fact, it's not meant to define anything. It's meant to objectively capture the ways language is used today, with the primary intent of being used as a tool to help people understand things. If you read or hear a sentence containing a particular term that you're unfamiliar with then a dictionary will help you to understand the intended meaning of the sentence.
Acronyms, idioms, compound terms, and even punctuation are just as necessary for understanding as standalone words, so there's no reason not to include them.
When you put two nouns next to each other, it's not remotely obvious what the relation between the two is unless you just already know.
Similarly, alt-Earth's "bit rot" is an equine gum disease, "air fryer" is a slang synonym for a smoking-fast fastball; and so on.
Anyway, part of the usefulness of dictionaries is to preserve today's meaning for posterity, when what consider to be obvious context may no longer be widely known. The Dictionarium Anglo-Britannicum, one of the earliest English language dictionaries from 1708, included compound phrases like the "Ropes of a Ship," which meant pretty much exactly what you would think it meant, but also phrases like "Equivocal Generation"[1] which might sound to contemporary ears like it's referring to hipsters or something, but actually means something totally different.
[0]https://dictionary.cambridge.org/us/dictionary/english/medic...