Which also raises the question why doesn’t the sorcerer supreme do the same thing for The Entire Planet
For one they are guardians of Earth, but it does not belong to them.
Also the Sorcerer Supreme is dedicated to protecting Earth from outside threats. The last thing they are looking to do in that process is suppressing the magic of latent sorcerers who could be brought into the fight. "No more sorcerers" is Mordo's cause - not Doctor Strange's cause.