Can't do that: the pilot can't be allowed to control, or even know about MCAS, because that would affect Boeing's ability to claim the plane is "just another" 737 and that the pilots don't need retraining. Giving the pilot the ability to control MCAS means the pilots all need retraining.
That's certainly the regulatory logic, but I feel like the entire idea of hiding critical systems from the pilots may have been a bad idea from the beginning. Regulators should never have even entertained the idea of using active compensation systems to maintain the type class.
They already received training on MCAS once they got 737 MAXes, albeit a short superficial one (1 hour of lecture&1 hour of simulator training iirc). Hence "cannot be allowed to know" does not apply anymore.
Yeah, and that's wrong. It should apply. Pilots should not be allowed to know, because that means this is a different plane than the one they were rated for. And so, since the plane can't be flown safely without proper training, the planes should not get the same type rating as the old 737, and either 1) they should just be demolished, or 2) they should be considered an entirely different plane, with all the training requirements that entails. The regulators completely failed here, and by allowing this are showing they're corrupt.