The main thing is that they were only barely part of France anyway. They were essentially independent rulers whose recognition of royal authority was mostly nominal.
And of course, within a few generations, the Duchy of Aquitaine did break away (as Eleanor of Aquitatine dumped the king of France and married Henry II of England to form the Angevin Empire, where parts of it would remain until 1453). Meanwhile, Toulouse didn't go quite as far, but it did cultivate strong ties with the crown of Aragon, until it was conquered by more reliable northern French lords during the Cathar Crusade.
They weren't alone in that, of course. William the Bastard, before he decided to invade England, had already established a formidable military reputation beating up on his neighboring French vassals and even the King of France, Robert of Burgundy had basically forced the previous French king to hand over his duchy, and Flanders was a perpetual bone of contention between France, the HRE, and later England.