I am/was in a similar situation. Getting better is such an insurmountable thing when you're there. When things have been so bad for so long that they're not just normal but comfortable in a terrible way, just telling a friend about it is extremely difficult to do. Even if you can gather together the will to try to seek out professional help, you probably won't keep it long enough to actually make that far. And, some part of you that you're desperately trying to keep quiet is telling you that if getting help is that easy then you've just wasted your life laying around being helpless and useless and shitty when it could have been better and that's something else weighing down the "kill yourself" side of the scale. You've been accepting it and coping with it for so long that it's the only future you can ever see yourself having.
I think a lot of them don't believe therapy can change anything about their situation.
They don't think it has anything to do with their mental health. They believe that them being lonely, unwanted and unworthy of love is just the natural state of things and all they can do is learn to cope with it. And as long as they can function and get by most days, that's as good as it will ever get for them. So in their logic, therapy makes no sense because there's nothing to improve.
The very low default state of being is very accurate. It's how you survive when things are that bad, you can't hope things will get better because that won't happen and it will only push you lower than you already were when you inevitably give that hope up again. You are incapable of being close to people, because it's just not possible to support you. You'll just end up dragging them down. Trying to find a relationship is just not something you're capable of, and yet you still resent the loneliness. You either blame yourself and the depression gets worse, or you blame the people you couldn't have a relationship with anyways because you can't try and really go down the road to inceldom.
It's not like most people going through this are unaware of their mental health problems. It's just that the thought patterns you're stuck in keep you stuck in them. Getting help seems like a herculean task, and you will never have the strength to attempt it.
Probably the biggest one is cost for therapy (or waiting lists, scheduling, paperwork, transportation/distance, or thinking it's not gonna do much especially if they have other untreated issues).
And get help where? I have different issues which I even have a proper diagnosis for and even then no therapist will even put me on a waiting list. The few times I have gotten in to see a therapist I only got a few sessions before they all basically started shoving me out the door. I'm mostly functional and not actively suicidal so I'm just not a priority. They all have people with worse issues that need to be helped so I'm at the bottom of the triage list. OOP is working, excercising, and socializing. They're even more functional than I am. No therapist is going to so much as give them the time of day if my experience is anything to go by.
I don't know in what state psychological help is where you live, but today the are more options for that than ever before. But yeah I'm sure it's difficult.
I'm in the US and actually in a state where medical care is literally world class (I live less than an hour from the Mayo Clinic) so I'm probably even more lucky than most people as far as options go.
A lot of the issue is what insurance will cover. I can't afford to pay a couple hundred dollars out of pocket every week to talk to a therapist. But the options my insurance covers also just so happen to be the options everyone elses insurance covers so they are all booked solid with people who are in far more critical condition than me. In general there just aren't enough therapists to go around so those that are available need to focus where they are most needed first. I get that. It's literally no different than hospital triage. It's just hard not to see it as being told that your issues aren't bad enough that anyone cares. Or even worse, maybe you're just faking it.