a) because services like Twitter and Facebook have to show something in the timeline. What they show is this job
b) because in practice, an approach too laissez-faire here has negative impact both on customer adoption (most people don't actually enjoy cesspools, and they leave when they decide a service is one) and regulation (even in countries with broad free-speech protections, some communication is de facto illegal and there are consequences for a service taking no responsibility for keeping their house in order).
It's called a search engineer, and this isn't left to people's own discretion because they don't have the time or inclination to make their own search engines / recommendation engines.
If there's someone we can't trust to kill/harass/assault people, they need to be in prison, not having speech policed for them as if they were a child. That's one of the core purposes of imprisonment, in fact.
But the reality is that we can, in fact, trust most people to not do those things. And yes, that means sometimes we will be wrong and we will have to pick up the pieces after someone does something awful. That is simply part and parcel of living in a free society.
The people being threatened disagree with you. They aren't living in a free society. They are forced to protect themselves, both in real life and online.
Personally, I'd be in favor of enforcing existing laws about threats. Make a death threat, go to jail. Law enforcement is pretty bad at that, with the excuse that most death threats turn into action, and it's very difficult to track down an anonymous commenter.
But I'd like to see what would happen if they took the existing laws seriously. Maybe then I'd find it easier to credit the notion of unrestricted free speech that doesn't rise to the level of criminality. I'm not convinced, but I'd at least be able to consider it.
Meantime, "Somebody threatened to kill you and that's your problem until you're actually dead" does not seem like an acceptable situation.
b) because in practice, an approach too laissez-faire here has negative impact both on customer adoption (most people don't actually enjoy cesspools, and they leave when they decide a service is one) and regulation (even in countries with broad free-speech protections, some communication is de facto illegal and there are consequences for a service taking no responsibility for keeping their house in order).