That's more the exception than the norm with most of the writers I've listed, but you also have to consider my culture is not yours, and the French literary tradition is (and even more, was) quite different from the anglo-saxon one. French and English for all their similarities remain very different languages, and have different aesthetics.
Most of the writers I've listed are hardly florid and in fact some like Zola, Camus are recognized for the very opposite!

As for "poetic", this has a slightly different value in the French language (and it's not exclusive to French... Japanese, Russian, Chinese... many languages values the poetic aspect of prose a great deal more than it's done in English (except of course in poetry in its various forms), that values "efficiency" over beauty in prose. By "poetic" the French don't necessarily mean the language is close to the aesthetics of classic poetry. It's more the combination of how interesting it sounds aloud (the musicality, the rhythm), and how evocative it is.

As an aside, that's hardly the sole standards by which French culture has traditionally judged what is and isn't "great literature", especially foreign ones, for which the French just don't expect French aesthetics and are far more interested in substance, vision, perspective for foreign books (they value that a lot in French literature too, of course - it's not all or necessarily primarly about "beautiful prose". The French don't like beautiful prose without substance (but do say that writing great ideas with average prose is a shame!).