It's a bureaucratic problem. Getting a headcount approved for a specific role is a non-issue. Getting HC approved for something brand new might be impressively complicated. Especially at large companies, listed postings persist for many months sometimes and stagnate because functions, dynamics or org alignment shifts before anyone is hired. It's much easier to write a generic-ish job posting and just coach the recruiter/screener on updated specifics than it is to retract a posting, rewrite it, and repost it. This happened to me frequently when I was a hiring manager at Google, for example.

This works for some orgs but not for ones where roles require significant negotiation and sign-off. That can take weeks or months and by that point an attractive candidate has moved on.

To be honest, the places I’ve worked for that could do this were not the best jobs I’ve had. That flexibility can sometimes point to other organizational issues.