Probably because back then people actually talked about the game or something.
Back before I lapsed out of Multiplayer, I mute people for:
- Singing into the microphone
- Blasting music into the microphone
- Talking about their medical problems (ew.)
- Using it as a telephone, airing their conversation throughout the game
- Random swearing
- Being jerks in general.
Any COD or Battlefield game is this. I only have it on with friends or any cool people that makes it all worth while.