If masks are so effective in combating covid, how come so many mask wearing people are being diagnosed with covid?

If this is an airborne disease, why are the masks not stopping it?

If mass infection leads to herd immunity, why is it mandatory for everyone to wear a mask (and assuming get the vaccine)?

If seeing one's mouth and facial expression leads children to learn immeasurably better than when those things are unavailable, why are we covering the faces of teachers and young children in schools when this population is also at the least risk for contracting covid?

I just have so many questions!