Well, because he doesn't feel like it. We know he isn't above interfering with free will. Exodus 7:3

“But I will harden Pharaoh’s heart, and I will multiply my signs and wonders in the land of Egypt. ”

Exodus 14:4

I will harden Pharaoh’s heart, and he will pursue them, so that I will gain glory for myself over Pharaoh and all his army, and the Egyptians shall know that I am the Lord.” And they did so.

Dude literally contravened Pharaoh's free will just to flex.