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.