I agree with your point. However, I think it applies to youth more so than adults. I strongly think that a lot of adults who suffer these type of hardships are aware of them. And have tried to remedy them.
I know that for some: years and years of trying isn't enough to create neither type of social network.
I'm doing ok now, having started from near-zero network in my 40s, after a long lull and letting my social and professional circles fade away due to life circumstances.
That's why I say it can be done when older too, and have an idea what the timescale is.
(I also think the "other people appreciate your potential value in their network" point applies more to older people than younger. That is, if you're older, your perceived network value is a bit higher; I'm guessing the age is vaguely associated with seniority and future opportunity for the other person.)
However, I have the advantage that I did figure out people and developing and maintaining social links (if I want to and feel up to it), a number of years ago. I've led community groups occasionally too, which is not always pleasant but very educational.
I still remember not knowing how to, having no active friends at times, and being very lonely and isolated. So I empathise (a lot) with those who are stuck and unhappy with that.
For those people you're talking about who have tried for years and years, if they're really stuck, I wonder if there is some kind of coaching or training that would make a useful difference, assuming they want to change the situation. If there isn't, there ought to be.
I know that for some: years and years of trying isn't enough to create neither type of social network.