Not the OP, but here are a few reasons I've seen:

- the boss has agreed to the role but has reservations, seeing a few candidates solidifies them and permission to hire us withdrawn

- the team is inexperienced at hiring and don't know what they want until they've seen a few candidates

- the company is hiring a new whole team. To make hiring easier, roles that are listed are "representative roles" - the total desired skill set across all roles is accurate but the company doesn't care what the split is, they just want a team that covers it. So a candidate who is a better fit for a listed role can be passed over in favour of one that happens to be the right jigsaw piece.

- circumstances changed since permission to hire was given, and no-one remembered to update the hiring portal; because unless you're actively hiring no-one looks at it.

This last one is quite common, because there are so many applications usually that no-one wants them in their email.

A company has a graduate scheme, they might be hiring 4 graduates this year, they might be hiring 40. Only one job advert.

External recruiters might then re-advertise the job with the company name removed, planning to funnel people to the company and collect their 20% commission.

External recruiters with several jobs might merge them into one. $250k job for a senior java developer with 5 years finance experience + $75k job for a junior java developer = advertise $250k job for a java developer.

A company might have a slow, centralised hiring pipeline for some roles. Google has a recruiter check the candidate's resume before putting you into a lengthy 6+ interview gauntlet, but only at the end of it do hiring managers actually check if the resume matches an open job. And if course if it takes 2 months to get through the full pipeline, the jobs open at the end aren't the same as the jobs open at the start.

I might need four different (distinct) roles filled but only have budget for three employees. I’d be prone to advertise (and genuinely recruit) all four roles, and hire the first three that I find a good fit for, delaying the fourth until/if additional budget frees up.

I might be willing to hire for two different levels of seniority/experience, but only one xor the other, not both.

Or be willing to hire for a role in Boston, London, or Zurich, but only one of those.