RA knows...

There is a reason RA wants Seth to tell the truth. There is a reason why every other God and Goddess dosn't try to search for the truth. And there is a reason why Seth has always been in agony.

Or: Seth has been cursed right after his birth by Apep ,along with the other God's and Godnesses and RA can't do anything as long as no-one breaks free from the curse...or is consumed by Apep's curse.