Because some people LIKE to come across differing opinions.
Some people prefer having an open marketplace of ideas, where they can be challenged, and come across opinions and ideas that they wouldn't otherwise in real life.
Absolutely, but I think you're being a bit naive. I like to hear different opinions (I like that /r/ChangeMyView is a thing), but there's a difference between reading a different opinion and reading "Kill yourself, faggot".
If you advertise being a place of COMPLETE free speech (Like Voat), you're gonna get assholes that think free speech means having a place to be an asshole.
And some people have a thick skin and are fine with others insulting then.
Places like 4chan are fairly popular, even if the discourse is often mean. Sometimes things can be so mean that it is simply ridiculous, and therefore funny, and therefore not really insulting or offensive.
Some people prefer having an open marketplace of ideas, where they can be challenged, and come across opinions and ideas that they wouldn't otherwise in real life.