It’s already infected with the bad bacteria, and is a great growth medium for them. Also not everything you buy has bad bacteria or only has bad bacteria on the outside. Like you wouldn’t expect fresh fruit to have bacteria inside it because it’s a living thing with the ability to defend itself. Cooking food kills most bacteria too; that’s why pasteurizing things works.
Raw milk is known to be a major health risk, why would you just ignore it? Like you wouldn’t eat raw chicken so why milk?