So Joseph made it a statute concerning the land of Egypt, and it stands to this day, that Pharaoh should have the fifth; the land of the priests alone did not become Pharaoh's.
Genesis 47:26

The people of Egypt were taxed by Pharaoh based on his asserted prior ownership of them. This is the same reason that the priests were not taxed. The priests belonged to their respective deities, who were supposed to be peers of Pharaoh.