Because it's SOOO difficult to invite someone to a party or fireteam to talk.
After years of mmo's I can safely say people in public chat have nothing useful or interesting to say.
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Well I guess it's a good thing that it isn't an MMO.. But honestly, public chats on console seem to be more bareable, worst chat I have seen on consoles is GTA Online... other than that, people are usually quite or don't even have a mic connected. So I can find it easier to simply mute someone than be overwhelmed by people shouting in chat like wild animals as some are assuming.