Strong AI could be impossible (in our universe) if we're in a simulation, and the software running us combs through things we create and sabotages every attempt we make.

Or if we're not really "strongly" intelligent ourselves. Invoke absolute denial mechanism.

Or if humans run on souls which have access to some required higher form of computation and are magically attached to unmodified children of normal human beings, and attempting to engineer something different out of our own reproduction summons the avatar of Cthulhu.

Or if there actually is no order in the universe and we're Boltzmann brains.