Two products result at the end of the butter-making process: butter, and the liquid left over, which is called Buttermilk.
Butter in North America used to mostly be cultured butter (see entry for Butter). The Buttermilk left over from the butter-making process would carry with it both the cultures used for the butter as well as the distinct tastes of the cultures. The Buttermilk would essentially be skim milk, with all the fat used up for the butter, but the continuing action of the cultures in the Buttermilk would make it the thick, tart milk that people loved.
In the 1940s and '50s, however, North American butter-making switched from making cultured butter to making sweet cream butter, which didn't use the cultures that would make produce the Buttermilk. Consequently, what North Americans now know as Buttermilk is actually skim milk to which a culture is added that creates lactic acid in the milk. It yields a taste and texture similar to what Buttermilk used to be. But because it hasn't come from the butter-making process, it is missing the smooth, emulsifying properties that had come from contact with concentrated butterfat, that had made real Buttermilk a cherished and relied-upon ingredient for baking and cooking. So when you see it called for over and over again in old recipes, that's why they relied on it.
The other reason we are now sold "artificial" (or "cultured", as the marketers call it) Buttermilk is that relying on a side-product such as Buttermilk really couldn't meet market demand. One gallon of milk will make 3/4 pint of heavy cream (40%), which can be used to make 1/3 pound of butter and only 1/2 pint of Buttermilk. The other 7 1/4 pints of skim milk, though, can be used to create Buttermilk artificially by adding a culture to the skim milk lets you have another 7 1/4 pints of Buttermilk, which can be sold at higher prices than skim milk.
But there's no point crying over unspoilt milk: even the weaker Buttermilk that we have today as a result can still give a rich, tangy flavour to baking. But it isn't the same milk that our grandparents would relish a glass of.
Substitutes: To one cup (8 oz / 250 ml) of milk, add either 1 3/4 tsp of cream of tartar or 1 tbsp lemon juice or 1 tbsp white vinegar. Let stand for 10 minutes. Or, try 2 parts plain yoghurt or sour cream, plus 1 part milk