Didn't 4chan start off largely as castaways from Something Awful? Your point is valid though. 4chan had an enormous amount of time to become the shining star of how great an absolute[1] free speech site could be, but still manages to be a cesspool. This should be a neon hint, but people keep thinking that they're going to invent the one free speech site that doesn't end up toxic.

1: Also as others point out, even 4chan moderates, however lightly.

4chan is not unmoderated. The moderation is somewhat hidden and the levels of acceptability is lower and different, e.g an unsophisticated hate speech post calling for explicit violence won't be allowed and the user banned quickly, however a more insidious and hateful post about a group (without the call to violence) would be allowed. I think it uses usa speech law as it's basis. There's a theory that they proactively work with LEAs to report infringing posts and users to keep itself running - basically a kind of honeypot.

It also has troll pollution, something which is not moderated out and makes it more toxic and not a healthy place at all.

The moderation rules differ according to which board the user is on instead of apply to the whole site. Even the ones which are not adult themed still have trolls.

And 4chan got much worse over time, there's no reason it had to be rejects at all. The toxicity was entirely self directed.

…or capturing the market would have flooded it in inane speech, rendering it no better (or different) than anywhere else.