Cows milk can vary in levels of fat content due to the breed and location. In the past, yes milk just would have been what was left after the cream was taken off, but even without that cream it is still pretty creamy, nothing like the watery stuff you get with skim milk other low fat milks. If we skim the cream off the raw milk, the milk left behind is creamier than the full fat you can buy. The milk that was delivered when I was a kid, still had the cream layer.

These days all milk is highly processed. It is basically pulled apart and then put back together in different ratios to make milk of varying types. Skim milk powder is still very cheap though considering the changes in processing, they are just off loading a by product.