People are purposely doing it to be as annoying as possible.

I'm not sure the devs are totally aware of this, cough, but giving the ability to be annoying twats with no consequences people make that choice.

Look at these assholes. LOOK at them, and they started teamkilling me every spawn when I told them to shut up.

https://imgur.com/a/ZK0GkSc
and these are their Steam profiles in case anyone actually cares.
https://steamcommunity.com/profiles/76561198082551864/
https://steamcommunity.com/profiles/76561198129720791/

People are also saying a lot of inflammatory shit because they know no one can tell who is talking. And the mute feature doesn't work.