If it doesn't exist, why hasn't someone started doing it?
Because it ain't worth it.
The ad manager either needs several accounts, each with a 5-figure monthly spend, or hundreds of piddly accounts.
The former is tough to make happen because there aren't a ton of authors spending 5 figures a month on FB. The latter is tough to make work because the ad manager becomes overwhelmed.
We saw the latter happen last year with a service targeting AMS ads.
I agree completely. If it were worth paying someone to do, that's what more of us would do. And when you discover it's not worth paying someone to do something for you, it kinda makes you wonder why you'd bother doing it.
That being said, there are some situations where having an ad agency working for you would be worthwhile. For example, if the agency is handling a few authors in similar genres and can run collaborative ads with a group of books.
This entity might take a book and keep it between 500 and 15 in the Amazon store for 12 months, occasionally dropping the price as needed for turbo boosts.
That same entity might also be running high-budget constant ads for similar books, thus giving a multiplier effect to the group of related books that are populating each other's alsobots. Advertising on one title might not convert to the target book, but could spillover to one of the agency's other titles in the alsobots.
THIS IS A THEORY, PRESENTED FOR ENTERTAINMENT PURPOSES ONLY.
ETA: Everyone's mileage with ads is going to differ by genre ad product. I had a terrific run with FB ads back in 2013. It was amazing. 5 to 7 cents a click, like shooting fish in a barrel, and I sold a ton of $7.99 bundles. My rank was between 300 and 800 for months, and I raked it in. This was before the KUpocalypse, mind you.
What's nice about FB ads is they do give you the sense that you have some control over your career. It's a big mood boost when you feel like something is within your control.
However, the costs keep getting higher, and I've found it's much harder now to sell boxed sets, and the first-in-series sales haven't converted well for me. YMMV. I was selling a lot of 99-cent books to impulse buyers, but it ended there. It's likely my series is too niche or the book was too long or too short or didn't have a compelling cliffhanger, but I'm probably done with FB ads unless there are 3 more apocalyptic events and I have to try something out of desperation.