Exactly those with a huge social circle, career, and life partner don't care about any of this
This is exactly right. Most people stay very busy and don't have the time to heavily contemplate these topics, or they are able to quickly bounce towards a distraction if uncomfortable thoughts arise. For people with happy, fulfilling lives, they are typically engrossed in some sort of activity, and thus these unsavory things can't permeate their minds.
If you have too much time on your hands or become sendatary for whatever reason, you seem to become more prone to ruminating over the bad things in this world, unless they are imminently staring you right in the face. As a child, I would have never even known what suicidal ideation was if my father had not pulled out a gun and tried to shoot himself in front of me.
Likewise, I wouldn't have been inclined to think about all the unsavory aspects of life if I didn't spend most of my existence in bed due to health problems. Engaging with the world in productive, satisfying ways is the only way you can hide from sinister realizations, but often the ones who need that escape the most are unable to partake in it for whatever reason, which goes back to your original question.
I don't think every single user of that subreddit had the capacity to know their loved one was suicidal, either because they didn't have enough time, they didn't take it seriously if they were aware of it, the other party hid it from them well, etc. However, from a cursory glance, I don't think that's the story of the majority.
Many of them were aware that their loved ones were suicidal, however, they were powerless to stop it. I noticed that there is a tendency to say the other party had a "diseased brain" or weren't "in their right mind" but you can tell from a good number of the anecdotes posted on there that many of those who passed on had been suffering for years and years and nothing was attenuating their pain. A far cry from a temporary crisis yielding from an unsound mind.
This attitude is not a fault of those who are grieving, but a consequence of the mainstream narrative that every single suicide is preventable and it cannot ever be a rational decision. One comment even says, ".. But suicide is not like euthanasia. Suicide is an act of extreme violence towards themselves and their immediate circle."
I don't think you could change the mind of someone who holds such a strong conviction about suicide being an extreme act of violence, that it is often a choice one makes after exhausting all other options, and in most cases, there is no underlying maliciousness held towards those in the person's immediate wake.
Fundamentally, I don't think those people can understand unless they have been suicidal themselves and experienced a change of heart. To many of the people on that subreddit, every suicidal person is mentally ill (with depression, as no other mental illnesses seem to ever exist to onlookers) and if only they had taken enough pills and sat through enough therapy, they could've been cured.
I have even seen some posts on there where grieving individuals want to sue other family members or friends for advising their late loved one to stop taking medications or ceasing contact with services they didn't find helpful, prior to their suicide. Unfortunately, many of life's problems cannot be fixed with a pill, as wonderful as it would be if this was true.
I don't think it's possible for people to truly understand the perspective of a hopeless suicidal person when they have no experience with failures in healthcare institutions, and cling to this idea that everyone who experiences suicidality has temporary depression, when this is a false generalisation. The reality that there are many things that would drive someone to ctb is extremely uncomfortable no matter what your belief system is.
