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or is it a loan phrase..? I only ask as "raison", "de", "être", are 3 words forming a phrase, non?

You can certainly call it a loan phrase. I suppose it would be more precise to say 'loan concept composed of two or more words'. In English as in French, many combinations of words, like 'loan phrase', really count as one word (e.g., 'prime minister' or 'tax evasion', to pick two examples completely at random and out of thin air) even though they're composed of two or more words, if you know what I mean. :)

Of course, in German the component words of compound nouns are generally run together without spaces (and using a couple of rules), and there you can usually say that several words are turned into one new word without encountering the above problem. (There are exceptions even in German.) It just doesn't work in English spelling to spell words like that 'primeminister' or 'taxevasion', for various reasons. It also isn't really possible to settle on some other rule, e.g. more consistent hyphenation. I try to use hyphens more than most, but even with relatively low usage (e.g., 'web-site') it looks a bit old-fashioned and certainly can't be applied across the board.