Because if they're forced to act nice, most people will just quit. Nobody's nice all the time (everybody has problems), and online communities are great for when you want to vent once in a while.

Plus, all the nicety invites narcissistic assholes who think they are better than everyone else.

I've never heard of Imzy either, and I think I keep up with all the potentially interesting stuff online. Launched in 2016 and not a single time I've even read the name.