Background noises happen. People take notes using a keyboard.
People cough. If it's a few people interactively chatting, sure, leave things unmuted. In a big group call where someone's presenting, please mute however leet your audio setup is.
Well I use a an actual Shure dynamic mic and unless you get close 3 inches or so it pics up nothing - this is not you crappy laptop mic or 19.99 headset.