I believe the answer is: because pasteurization doesn't sterilize milk. It only kills most spoilage microbes.
Because of this there is still left over in the product after the process. The process only slow down the rottening , but it would not stop it. The bacteries that targeted by the pasteurization usually only the one that harmful. The most likely effect of consuming such outdated milk would most likely only sour taste on your tounge.
