Because the so-called idioms are fixed phrases formed by long-term use and tempering in language, which are larger than a word and have grammatical functions equivalent to a word's language unit. Most idioms are composed of four words, but they are not limited to four words. Most of them come from historical stories, fables, myths or other legends, as well as classical literary works.
For example, "cut the gordian knot quickly", "do your best", "the donkey's lips are not right for the horse's mouth" and "fear the wolf before and the tiger after". These words with more than four words are also idioms. And the supremacy of love is a word.