Urban legends work super well on TikTok because it's the perfect format for it: short, visual, and you can stack testimonies or "proof" in video form. It's less of a return to folklore than a transformation of folklore. Stories about Slenderman or haunted houses existed before, but now they become collective in real time. A teen posts a story, a hundred others add details, local versions, "I saw the same thing", and boom, it's become a shared legend live. It's contagious because it creates a sense of community and secrecy among those who "know".
What's changing too is that before you might've heard a legend around a campfire, now it spreads like a meme but with an aura of "it might actually be true". TikTok amplifies the ambiguity between fiction and reality, which makes the whole thing even more freaky. And the algorithms love that, so the more you watch, the more you see "horrific" content, the more you want to participate.
One thing teens don't always do: check the sources or see if the legend exists elsewhere. If you search the name + "snopes" or "knowyourmeme", you often discover that the "true story" has been circulating for years and everyone just reinvents it slightly. It's actually a pretty good critical thinking exercise, rather than just swallowing the content as is.