Maybe because "grande école" literally means "big school", but it's essentially a proper noun in French, similar to their version of "Ivy League"? Not sure why

Maybe because "grande école" literally means "big school", but it's essentially a proper noun in French, similar to their version of "Ivy League"? Not sure why for the others you mention, except that they're very specific concepts in France as well, which don't really have very good analogues in the US. Or maybe they just like the sound of the French word and want to sound more sophisticated.