Why does pasteurized milk in an unopened container spoil after several weeks

Why does pasteurized milk in an unopened container spoil after several weeks even though it has been properly refrigerated at temperatures under 4 degrees C during this entire time?

Please help Biologists, Chemists, and Biochemist.
Thanks!!!

Pasteurized milk is not sterilized milk. There are still plenty of bacteria in there, just no pathogenic ones. And refrigeration temperatures do not stop bacterial growth, just slow it down.