Because that's already done on Twitter; it would be quite boring to pretend that it's something taboo.
"White people have poor taste in food!" isn't very punchy, is it?
They tell jokes about white ethnicities, but don't speak unironically, constantly, and at length about why whites should be racially exterminated like they do towards non-Whites, Jews, Muslims.
I'm not saying all this to imply that /pol/ is even handed but they simply don't talk like "white people have bad food! Lol they season turkey with water!". The entire self justification for their behavior in many cases revolves around believing others hate them as much as they hate others, and people are liable to spread anti-white hate speech just to agitate.
That other stuff? That got you mad. You've got multiple threads running about how terrible it is - and there's a good chance you don't even identify as one of those groups.
But let's assume for a moment that 4chan really is concentrated evil. You do realize you are basically advertising for them right now?
Does one have to be non-White to be opposed to the mass murder of non-Whites? Shouldn't every non-racist psychopath White person be offended by the idea of mass murdering non-Whites?
I see plenty of posts attacking and calling for the killing and complete extermination of non-Whites, Jews, homosexuals, and women all over the place but 0 posts attacking straight white males for being straight white males.
Gee, I wonder why that is? I really wonder who is posting all these white nationalist talking points, it's truly a mystery. Could be anyone.