Because it's a relatively safe level of risk compared to, say, 4chan's /b/. People love to dabble in risk in safe ways. I'd argue that's what many genres of fiction are explicitly for, e.g. horror, violent games, etc.
But it's only that way because Reddit placed limits which goes against the "let the internet be grimy" hot-take. Back in the day there were subreddits that would rival the depravity of /b/.
They never stayed contained I their own subs. They just needed to go out of their way to subs of their perceived enemies.
Yeah, we need something where the risk level is a little lower than the "it's a crime if I don't call the FBI and tell them I saw this" level.