Why grape juice? Why not grapes juice?

Well, "grape juice" is a compound noun (unhyphenated) used to describe the kind of juice.. strawberry juice, lemon juice, apple juice are all examples of this. If you are refering to the juice of many grapes then you can say "grapes juice".. "The grapes (plural) juice burst all over her blouse." Nouns used in the formation of compound nouns do not take the plural form.