"Girls, are you ready?"
Translation:Les filles, vous êtes prêtes ?
I think it's because without an article, there is no way to know the word is plural (in spoken French). And you wouldn't be able to use an indefinite article for specific "filles," so you use the definite article by default.
In everyday French, if you were calling to a group of girls, let’s say your two daughters, would you call out « Les filles » or just « Filles » ?