Because it's an idiom that has come mean something figuratively, not literally? Like "hot garbage". Plenty of info about the meaning of "dumpster fire" online.

Idioms don't belong in a dictionary though.

Why not? Since we're talking about definitions of words here, there's nothing in the definition of "dictionary" that precludes idioms. Indeed, having common idioms in a dictionary is useful, because you typically refer to a dictionary when you don't know something, i.e. you also don't know if it's an idiom or not. Better for the dictionary to tell you that it is an idiom (and what it means) than to simply omit it. People typically go to dictionaries to figure out the meaning of something they've just read or heard; whether or not something is an idiom, they still need to know what it means.

As English is my second language, I can say that reliance of English on idioms is enormous and they 100% should be easily lookupable.

Idioms are terms that have meaning distinct from their component parts that are used in a language; their omission from a dictionary would cripple the normal use of a dictionary.

Obviously the dictionary disagrees.