Not that these are prevalent in here, but here are a few reasons:
1. A company already has a candidate in mind for the role, and they want "plausible deniability" that they didn't try to fill the role through a competitive process. This is especially true with H1B-focused roles, where they have to justify hiring non-U.S. workers. Your resume serves that purpose.
2. They have an image to present. Shareholders have more valuable stock when the company appears to be doing well (dozens of unfilled positions) vs. not hiring at a given point in time. HN would be way less effective for this purpose than a company's own corporate website, but it's still part of the company image.
3. Workers are flaky as hell, and your rock star dev you snagged at 2x market rate just got an offer for 4x at a FAANG, and now you're left flat-footed.. OR.. you list their position as open, collect resumes, and as soon as they send you their two weeks you have a deep bench of eager recruits to interview. An unfortunate side effect is that you the applicant are highly unlikely to luck into the heir apparent role.
Although it's a big world, at the very same time it's a very small world. When I see that a company has an unfilled position which interests me, and I have applied, and I am qualified, and I haven't heard of them, then I will go to my LinkedIn account, ask around and speak my mind to people. A company that does it a lot won't get far. There are also some websites (I use GlassDoor) where you can comment on companies' 'attitudes'.