After milk has been pasteurized it is not possible for germs to live in it even if the milk is left at room temperature for a long time?

That is an incorrect postulation. Pasteurized milk is not sterile. It still contains microorganisms. But if you are referring to shelf stable aseptic pack, that product has undergone Ultra High Temperature pasteurization and was packaged in a sterile environment. Packaging has improved over the years, but even that packaging doesn't keep the milk forever. See Related Links.