Why is Pharaoh so obdurate? Why does he persist in the insanity of denying Israel their freedom even in the face of plague upon plague? It’s a good question, and one that, I think, the text itself suggests.
But, as is so often the case, scripture interprets itself. Look at Exodus 10, verses one and two:
I have hardened his heart and the heart of his servants, that I may show these signs of mine among them, and that you may tell in the hearing of your son and of your grandson how I have dealt harshly with the Egyptians and what signs I have done among them, that you may know that I am the Lord.
Well. That worked. The story makes it not only to the hearing of the grandson, but for innumerable generations, down to this very day.