I am not sure if this is true for France, but I have always had a suspicion that it is:
One reason I could conceive of for a country to decide to sanction and control the rankings for practitioners of foreign martial arts would be if public facilities and perhaps funding is made available to people for practicing martial arts. You might not want someone with a black belt from a belt mill to come in and say "okay, I am going to start using the gymnasium over there with the sprung floor to teach my students, I will need a couple of two-hour blocks a week." And whatever else they might demand, I am an American so my imagination does not serve me very well here.
But the point is, IF (and not sure if this is the picture in France, but I think it is something like it) the people decided to set aside a bunch of public funds for facilities for martial arts instruction, there would have to be some kind of official vetting process for people who were going to teach there. And maybe you'd want to do that INSIDE the martial arts system you were trying to support as opposed to externally.
This nails it.
The reasons why titles don't automatically cross, is because standards are different in different countries. I certainly wouldn't want a "doctor" automatically being granted the ability to call himself that, and operate on patients, simply because he got a "doctor" title in some random third world country.
Whenever a title carries responsibility with it, it's important to make sure that the title also lives up to the demands of that responsibility. In the case of Martial Arts, it can be to avoid injury of students, avoid teaching hazardous moves, avoid getting leeches gaining government funding, and so forth.