Because dohun doesn't really deem them important, he only sees yuwon. The face blur is temporarily lifted at the beach when yuwon is explaining why he believes there's good people, since hes actually looking at something other than yuwon for once even if its because he was told to. Its like how the brain filters out useless info, in this case he doesnt really care to memorize their exact features.

Can someone explain, why are his brother and father like that, i mean “black like shadows” and wtf is with that brother