Identifying the evil trespasser is a lot harder than you may think: the one making a shit post is only the tip of the iceberg, the real deal is the hidden portion of the userbase that approves of it.
In reddit terms, it's the people upvoting the shit post.
This leads to massive issues as moderation usually attempts to remove the visible issue, gets hit by a massive and completely unexpected backlash, and decides that clearly they were in the wrong and that the shit post is actually something good for the community.
A further issue is coordinated bad actors, such as the people organizing raids and moderation takeovers in their off-site secret clubs.
There's a reason reddit went to hell so fast, and it sure wasn't the administration.
Anonymous forums have less issues dealing with both attacks compared to standard forums, samefaggotry is assumed to be the default so it's hard to push change via short-term numberical advantage.