Because Q is someone very good at what he is about to teach who does not want to have his identity public. Dumblodore wants a decent teacher—as has been pointed out many times—and is willing to put up with a lot to get him.
Now weather Dumbledore knows the true identity of Q is a different question.
Because Q is someone very good at what he is about to teach who does not want to have his identity public. Dumblodore wants a decent teacher—as has been pointed out many times—and is willing to put up with a lot to get him. Now weather Dumbledore knows the true identity of Q is a different question.