The old "it wouldn't fulfill prophecy" excuse...?

can someone explain me why they think this is a legitimate excuse whenever something in the bible comes into question about jesus being god? example, if jesus was god, why did he allow judas to betray him? clearly, god would have known this was going to happen. the excuse given, this would not have fulfilled prophecy. but if prophecy is given by god to the prophets, why couldn't god have just told the prophets jesus would wise to judas' betrayal, and thus he would act accordingly to prevent being crucified?