Mammals drink milk when they're young. Maybe there are some exceptions to that, but the examples of milk-drinking mammals are pretty easy to find. People are mammals, and so the cow milk industry is sizable: the US alone produced over 200,000,000,000 pounds
of cow milk in 2013. In other parts of the world, goat milk is more popular (and there are a lot of other kinds of milk
that people drink, too). Okay, so why doesn't anyone drink pig milk?
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post