A compound word isn't just a phrase. The latter is a group of words that indicate a single concept. The former is a new word that has a distinct meaning from the subwords that compose it. "I love you" is an example of a clausal phrase. The meaning is entirely evident from the words that compose it. In contrast, a "hot dog" is not a particularly warm canine, and has its own OED entry [0] as a compound word.

And some of the entries on this list are wrong. "Good night" exists in OED as "goodnight" [1] because there are multiple ways it's used. One is the clausal phrase "I hope you have a good night", which can be modified by changing the adjective, e.g. "great night" or "terrible night". "Goodnight" the bedtime ritual can't be modified the same way, so OED chooses to write it as a compound word without spaces.

[0] https://www.oed.com/dictionary/hot-dog_n

[1] https://www.oed.com/dictionary/goodnight_n

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