Here's why:
* Traditional Uses: In some cultures, goat bladders (the organ that stores urine) were used for storing milk or other liquids. These bladders, though, are not considered a "sac" specifically for milk.
* Modern Practices: Modern dairy practices typically use containers like buckets, tanks, or plastic bags to store milk, not animal bladders.
If you're looking for a specific term for a container made from a goat's skin, you might consider "skin bag" or "hide bag," but these are general terms and wouldn't be specific to milk.