Soda streams are twice as expensive as buying fizzy water

New to Tattle Life? Click "Order Thread by Most Liked Posts" button below to get an idea of what the site is about:
I love fizzy water and try to reduce my plastic. But a soda stream works out at 28p a litre for mildy fizzy water and lidl sell fizzy water for 12p a litre for really fizzy water! How can that be? All that plastic and transporting heavy water is still half the price!