It was suggested to me that a PhD is more a test of perseverance than genius... I don't see why a prof would be reluctant unless they are not sure of themselve

It was suggested to me that a PhD is more a test of perseverance than genius... I don't see why a prof would be reluctant unless they are not sure of themselves and think that the system is somehow zero sum. Maybe the student shouldn't want such a prof as supervisor.