Because this is the mindset shitposters cannot leave even if they want to.
They have to kill any kind of on-topic discussion even if it's about something that everyone seems to love, because it gives them power. It gives them (You)s. It gives them the attention they desperately want and yet cannot receive in their school/home/circle of friends because they're absolute NEET-incels. If you notice, most of the rotten problem comes from /r9k/, and how when that was killed, we got the biggest increase of shitposting in all other boards. Now go track to how a site like Discord
is now linked to trannies because people from /r9k/ MAY (cause there's no legit source that I know of) have been tricked by assholes to get baited into getting their nudes in servers or some other insane stuff.
If you wanna get rid of shitposting, you gotta get rid of the cancer that plagues in boards like that one, where self-defeatism and attention-seeking gives you temporary comfort.