Even if people are drinking 4L of full cream milk, it still only has just over 1/2 the fat of 600ml cream.
4L of milk also has plenty of carbs and protein.
600ml of cream is basically all fat, only 12g of protein and 17.4g carbs.
Even just 1 glass of milk has the same carbs and protein content, but a tiny fraction of the fat.
Who wants to consume all that kind of fat from cream?
Why would you?