I am French so obviously not the best to discuss dialects but I would be curious to know what key reason would bring so many of them.

We have dialects in France, a few are very distinct but I would not call a dialect when someone pronounces a few things differently. I know that this is subjective, but still.

There are out course some mad places where they ("they" means, you know, they) call chocolatine a pain au chocolat (a French private joke, see https://www.legorafi.fr/2013/03/20/toulouse-il-se-fait-abatt... - in French from a leading national newspaper)