If you had a "smart" AI for hard and a "dumb" AI for easy difficulty, then you'd have to test and maintain two different AI modes. It's much more cost efficient to have an AI that works the same over all difficulties, and then penalize or buff it.
F.e. dumb AI never upgrades colony capitals to maximum, smart AI does. How does that affect gameplay and the AI's later abilities / possibilities? That's not really obvious. So you don't want AI to behave differently.
F.e. dumb AI never upgrades colony capitals to maximum, smart AI does. How does that affect gameplay and the AI's later abilities / possibilities? That's not really obvious. So you don't want AI to behave differently.