It really isn't. Napoleon conquered huge swathes of Europe, that became France after that time and a few of the Louis' conquered quite a lot too
Nice for example didn't really become part of France until the treaty of Turin in 1860 (it had a couple of French conquests before that for a sum total of about 10 years, prior to 1860 it was nearly always part of Savoy or Sardinia. Even now the locals speak Nicoise and the roadsigns are bilingual. Even after WWII they lost Saarland, which became an inependent country for 6 years and even played in the qualifiers for the World Cup in 1954, then it went back to Germany
France definitely hasn't been a remotely stable border since the 17th century, that is nonsense.
There's plenty more too, Alsace, Lorraine....