Why doesn't Arthur know that he is destined to be king

AMerlin has placed him under a spell in order to protect him from those who would do him harm

B)Arthur is extremely modest and though he secretly wishes to be king he would never admit it

CArthur's stepbrothers have kept him hidden in the dungeon of the castle most his life

Dhe doesn't know that he is ectors adopted son and that uther is his biological father