I wonder why God, when He wants to free His children from Egyptian slavery, chooses to have Moses tell Pharaoh to let them go to worship Him (Exodus 7-12), rather than giving some other reason?
That would seem like a direct attack on the supremacy of Pharaoh. He's the ruler, but the slaves want to go worship someone else. It's no wonder that he doesn't want to let them go. Not only is he afraid they will flee, but he also doesn't want to give an inch as far as his own ego is concerned.
Hard to blame him, isn't it?
Men will go to great lengths to protect their own power.