Multi-word terms get entries if their meaning is unpredictable in some way. For example, "red herring" should get an entry but "blue herring" doesn't, because it's just a herring that's blue.

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.