>>39527
>why change milk in the first place?
For fun and profit.
All foodstuff is similar. They add shit that doesn't belong to spare original ingredients with replacing part of them with something that would be discarded (like cornhusk) because they aren't food, but can be processed into something that is technically edible. It's like when drug dealers cut their products.
Milk is the same, they want to get the most out of it. Naturally there's not much cream on it, it has to be let sit a while. But after a day it starts clotting and more cream floats to the top, and when it is clotted completely you get a bunch of sour cream on top of it. Now they don't do this, they get as much cream out of it first as they can, because they want to sell more ("sweet") cream, and more sour cream. So they take out all of it, and they put back less. I heard the fat they add back is actually pigfat, but this info is practically gossip so I dunno.
Whey was consumed back then, even my parents drank it in their childhood.
Now they don't make cheese, but from the milk they extract casein, some of that they use for protein powders - similarly to whey -, but most they "build" cheese out of that. "Normal" cheese, and process ones too.