My guess is that it's to prevent cross-contamination. i.e. wearing a mask will protect a user, but the outside of it isn't sterile. So if you come off a covid

My guess is that it's to prevent cross-contamination. i.e. wearing a mask will protect a user, but the outside of it isn't sterile. So if you come off a covid ward, easier to just bin the mask and take a new one, than wash the outside of a re-usable one.