When farmers turned to mechanized milking at the beginning of the 20th century, some of the small farms got bigger, and production increased.
Here's the thing. Refrigeration didn't exist yet, unless you count the very ancient icebox, which you shouldn't. The icebox cost a lot to maintain and was inefficient -- because, as the name implies, it was basically an insulated box you put ice in, and ice tends to melt. And: glass doesn't insulate well.
This made milk going bad a problem. And you know what? No one had a good solution to the problem for a while! The best they could come up with was getting a milkman to bring fresh milk every single day and take away the empty glass bottles. This meant everyone always had fresh milk, but it also meant a family had to drink all the milk they ordered pretty fast, or else. It meant a lot of milk!