It's funny because I think both you and GP answered it? If the end result is almost the same, then... the reasons why one or the other is... Almost none really?
Or to put it another way, for historical reasons?
(I'm playing AC Unity right now... Set in Paris during the French Revolution... And you know what the contemporary city walls were used for? Tax collection purposes...)
This kind of stuff should be settled in trade agreements IMHO...
Which made me think about a future EU-US trade agreement... But guess what? Fucking Trump torpioed it.
You're not totally wrong, buy there's a really nice Delacroix not far from the totally underwhelming Mona Lisa / Joconde. (Yes "later", but still a must see.)
And I'm not totally sure, but I would bet that a lot of early medieval Christian art is French? I mean the Carolingians / Merovingians must have left something no?
And if the Louvre doesn't have impressionist art I would be totally disappointed? You're probably talking about the tennis court museum (edit: Jeu de paume museum, not far from the Louvre), but still? I think the problem is that the Louvre is so big than you're gonna miss stuff.
https://en.wikipedia.org/wiki/L%27Enfant_Plan