I looked up what "artificial" casings are made from and it says collagen and cellulose and it's used primarily in meats that are pre-peeled before they are sold, like hot dogs and deli meats.
Edit to add that I just read Johnsonville Brats are fully cooked, premium, coarsely ground pork sausages in a 5.75-inch beef collagen casing, already browned with grill marks and grilled for superior flavor.
Interesting. I'm wondering if there are any sausage manufacturers that use actual animal intestine.
Asking that question, here is the answer I found: No. Intestines were the original casings when sausages were first invented. They are sometimes still used in all natural butchers. These days most sausages are made with “Manufactured artificial casings are made of cellulose, collagen or synthetic materials.”
Makes sense, I suppose, for mass production. But those little corner butchers more than likely use animal intestine.