Marcel,because viruses are 80 nm wide. Not many cloths, even layered ones will block that. Not if you still want to breathe, that is.
So none of the virus will get caught in the cloth fibers of the mask? One Hundred percent of viruses will pass through the gaps between the cloth fibers...
Maybe that is true, but it seems unlikely to this layman. Is there some proof that is the case?
If not, isn't blocking any percentage of the virus from becoming airborne better than nothing at all? In this case, I know my sister the nurse would say anything is better than nothing.