I don't like backpacks. Mainly because I started developing back problems and I don't like the weight on my back. In addition I am getting too sweaty with a backpack. All our bikes have racks. Including the mountain bikes. So I am using paniers on my commuting and my mountain bike. And baskets on my old Gazelle.
The only exception: I have a small camelbak with a bladder only which I use very rarely.
But I guess it also depends how you grow up. Being from Germany I cannot remember that they ever sold bikes without racks back in the 70s except some street racing bikes and before mountain bikes came along. And even nowadays most commuting bikes are sold with racks and fenders over there.