There's a big difference between "I wish I could die" vs "I'm going to kill myself."

The first one is expressing an emotion.
The second one is stating an action.

People can help with the first one. They can empathsize, offer support and suggestions.

The second one - it puts pressure on people to have to "save" the other person and would lead to guilt and remorse and a lot of worry if they don't hear from the OP quickly. All we can do in an online forum is offer support and empathy - which we can give. But we can't save someone, and making people panic isn't fair.

It could be very triggering for the witnesses. There is also a lot of debate/evidence (can't for the life of me find it online right now but I'm running on veeeery little sleep) that shows how if say, a teen commits suicide then there is likely going to be a spike in that community because it affects everyone. Talking about feeling suicidal doesn't make someone else more suicidal... but actually doing it can.

And when we're online.. no one would ever know if that poster killed themselves, went and got checked in the hospital, or was just someone pulling a prank because they're an online troll. There is no way at all to know.

At least.. that's how I rationalize it.