And Joseph recognized his brothers, but they did not recognize him.
We may well wonder that Joseph, during the twenty years that he had now been in Egypt, especially during the last seven years that he had been in power there, never sent to his father to acquaint him with his circumstances; nay, it is strange that he who so often went throughout all the land of Egypt (Gen 41:45, Gen 41:46) never made an excursion...
Commenting on Genesis 42:7-20
And they said unto him, nay, my lord,.... One in the name of the rest, or each in his turn, denying that they were spies, and addressing him with the greatest reverence and submission, calling him their lord, and thus further accomplishing his dreams: but to buy food are thy servants come; that and no other was the errand they came upon.
Gen 42:7-8 Joseph recognised his brothers at once; but they could not recognise a brother who had not been seen for 20 years, and who, moreover, had not only become thoroughly Egyptianized, but had risen to be a great lord. And he acted as a foreigner (יתנכּר) towards them, speaking harshly, and asking them whence they had come.
Commenting on Genesis 42:7-8