The reason for that is the purging of threads that won't get bumped enough combined with the sorting by engagement not by actual votes like on Hackernews for example. This creates a high incentive to post controversial and upsetting stuff that will ensure the survival of one's thread and rewards trolls and extremists who are particuarly successful in employing that behavior. On small boards threads can exist for hours or even days. Whereas on large boards most threads will only last a few minutes, UNLESS the OP and the first few replies follow certain behavior patterns that have been proven to be successful for the survival of threads on large Futaba Channel type image boards. If the OP and the first few replies set the right tone for the following replies to follow, they will keep that thread successfully afloat.
The type of posts that will keep a thread alive on 4chan must be heavily transgressive, controversial, use in-group language and references that makes it easier to identify out-group "newf**s" etc, and be overall exciting and enagaging. It's tickling the short term reward system in the user's brains after all. This is how the mechanism of purging threads trains 4chan's user base to behave like the assholes of the internet. They are being manipulated by software design in an evolutionary matter. Nobody wants to spent time creating unsuccesful threads with little engagement that get deleted after just a few moments, it's frustrating, embarrassing and boring. So people who don't fit in either leave 4chan or adapt. That's why the site's "culture" and memes seems actually quite consistent and conforming (to only its own rules of course).
I find that's a huge mindfuck and manipulation of behavior, while not intended by the original designers of these types of image board software, it's still a thing to be aware of how these sites control their user's behavior. It's funny how this actually contrasts with the self-perception of their users as being based and elitist. They're actually in a huge online re-education camp that keeps them hooked via small bursts of endorphin.
The boards' topics have only in part to do with the toxicity of views and behaviors expressed on them. I propose the hypothesis that it's actually the limited amount of threads being kept at any single time that is a huge factor that increases evolutionary pressure on the user base. In result it's what rewards toxic behavior on sites like 4chan, and this board mechanism is responsible for training a subculture of young men to believe that this is acceptable or empowering behavior online. So they excert this learned behavior outside the mechanisms of image boards and also bring it into the real world and into politics where they are being used by political professional actors for campaigns of course since their previously trained behavior and thought patterns that get activated by those political manipulators are not based on reflection and introspection but on manipulation and getting rewards (in the form of "lulz". Thus "owning" your opponent takes the place of real intellectual political debates). Decreasing the amount of allowed threads and increasing the user traffic will both accelerate this behavioral re-education process in its users. 4chan and the like as a mass propaganda tool to manipulate a subset of the population is excellent! It's like an army boot camp for political trolls, if it didn't exist you'd have to invent and engineer it like 4chan.