Because of science.
Current guidance based on community exposure, for people exposed to people with known or suspected COVID-19 or possible COVID-19
Individual who has had close contact (within 6 feet for a total of 15 minutes or more)
Exposure to
    
- Person with COVID-19 who has symptoms (in the period from 2 days before symptom onset until they meet criteria for discontinuing home isolation; can be laboratory-confirmed or a clinically compatible illness)
 
-     Person who has tested positive for COVID-19 (laboratory confirmed) but has not had any symptoms (in the 2 days before the date of specimen collection until they meet criteria for discontinuing home isolation).
 
Note: This is irrespective of whether the person with COVID-19 or the contact was wearing a mask or whether the contact was wearing respiratory personal protective equipment (PPE)
https://www.cdc.gov/coronavirus/2019-ncov/php/public-health-recommendations.html
		 
		
	 
you did notice that they were talking about people who should quarantine themselves, right?  just to be doubly sure?  no mask or device can filter out ALL POSSIBLE VIRUS if you are very close to an infected person for too long...DUH...whoever claimed a mask was 100% effective?
from the same source- updated Dec. 31, 2020- (duh)
YOUR HEALTH
How to Protect Yourself & Others
Updated Dec. 31, 2020
Wear a mask over your nose and mouth
Masks help prevent you from getting or spreading the virus.
You could spread COVID-19 to others even if you do not feel sick.
Everyone should wear a mask in public settings and when around people who don’t live in your household, especially when other social distancing measures are difficult to maintain.
Masks should not be placed on young children under age 2, anyone who has trouble breathing, or is unconscious, incapacitated or otherwise unable to remove the mask without assistance.
Do NOT use a mask meant for a healthcare worker. Currently, surgical masks and N95 respirators are critical supplies that should be reserved for healthcare workers and other first responders.