There might be a colony of the virus on a surface, or in a few air-born droplets hanging out in the room. If your device didn't happen to sample those particular droplets or surfaces, you'd never know they were there.
True, there are multiple methods for transmitting the viral particles in question, but contact transmission would require that you actually come into contact with the surface in question. I don’t know what the exact numbers are, but I believe that contact transmission is considered fairly rare for COVID.
Moreover, having a partial solution to the problem is better than no solution at all.
