When a hiring manager needs a specialist recruiter, the search starts on Google - “IT recruitment agency London”, “engineering recruiters Manchester”, “finance headhunters”. Those client-side searches are made by people ready to hand over a retained brief worth thousands, and they're the searches most agencies never build pages for.
The reason why recruitment SEO pays back so well is that the client side of the SERP is genuinely winnable. Indeed and LinkedIn dominate job-title searches, but they don't compete for “best [sector] recruitment agency” - other agencies do, and most of their websites are a homepage, a generic jobs page and not much else. An agency with proper sector pages, location pages and proof of placements can take that traffic and keep it.
On the candidate side, the play is different: niche roles, salary guides and career content that job boards are too generic to win - which keeps your database growing without paying for every CV.