The fact that Greece modelled itself after France, as a strongly centralising state, is not a reason why there shouldn’t be autonomy. Autonomy can work financially, after all; the autonomy of the Val d’Aosta after WWII, forced on Italy by de Gaulle proposing to invade, was part of the reason the Valley did so well in the 60s: it could pursue and manage its own initiatives. And contra Ioannis Kokkinidis’ answer, I’m sceptical that centralisation was a spontaneous demand from the people resenting feudalism, as opposed to an imposition from on high.
As Joe B’s answer points out, there is in fact some decentralisation and political autonomy now that wasn’t there before. Being much larger, the regions are more capable of getting things done than the prefectures used to be when I was a kid.
The real reason of course is Greek anxiety about secessionism, which you can see reflections of in Niko Vasileas’ answer. There has not been any serious talk about secessionism; in fact, outside of a fringe in Crete, there hasn’t been *any* talk that I know of of secessionism. But notice Greeks’ reactions to such hypotheticals: not laughter, but anger.
(Imma just leave this here: Is 2012 The Year Cretans Decide If They Want Out (from Greece)? And no, there was no expiration of a treaty joining Crete to Greece.)