Have you ever come across a recipe that uses more than a cup of buttermilk? Because I haven't. And I bake all the time.
Every other specialized dairy product is sold in sizes from the pint, if not the half-pint. Because you rarely if ever need more than a cup or two of, say, heavy cream, and shops have figured this out.
But here I am, stuck with a cup and three quarters of buttermilk in my fridge, and that's after both scones and pancakes. And while there are worse things than being forced into a week-long bonanza of scones, pancakes, and biscuits, it's still unnecessary and foolish.