Because it's a fiction. Germany has never been just "German", likewise for France. They've always been a hodgepodge of ethnicities and languages and dialects, and destinations and sources for migration. It's only in the modern (17-18th century and on) that you start to get this narrowing of definition and some attempt to actually eliminate diversity within (rather arbitrary) political boundaries. ("The language of the Republic is French." aka f*ck you Breton, Occitan, Alsace, various other dialects of French, etc.)

If you pan the camera out over history, it's a bit absurd.

FWIW my Oma was Alsatian, German (dialect) speaking and my father was born in France at the end of the war... but is German.

For anyone claiming any European country is "originally an ethnostate" I recommend zooming in a little and looking at cultural and language minorities, especially endangered languages. In many cases even people born in those countries aren't aware of all of them and would consider at least some groups "foreign" despite them having been around for hundreds of years and often only recently marginalized. This seems to be well-known in France and Spain thanks to the existence of separatist movements but the only thing people usually seem to know about Germany is that some people in neighboring countries speak German dialects because the borders changed throughout history -- and it's far weirder than that. Many of these languages go back to "travelers", which in modern continental Europe is now only associated with Roma and Sinti peoples.