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boss has agreed to the role but has reservations, seeing a few candidates solidifies them and permission to hire us withdrawn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