Because this isn't reality, it's fiction, and people turn to fiction because reality is shitty. No one likes it when perpetrators get off free irl, but at least in fiction you expect it to happen. No sane person would read a realistic outcome of a story like this (and realistically, it would have ended with Siwon offing himself, but the author went to the trouble of transmigrating Taemin into his body, basically turning it into a fantasy, and the biggest perpetrator gets no punishment???)
If they prefer to read fiction to escape reality, why would they read red flag or toxic stories in the first place? That's their choice. Some stories are meant to be realistic, that's the intention from the author if that's the point.
I don't get why some people think the perpetrators NEED to get punished when in reality, obviously, they never suffer from consequences. Not all villain are punishable, some are purely evil and they have supporters around them. Some victims prefer to move on and want nothing to do with them, as for their best (mentally and emotionally) and it will be more torturing to meet back the people who broke/hurt them