Now that YouTube and other platforms have become so diluted that ad revenue is minimal, it makes sense that anyone with a large following would seek an agency to help them find advertisers. I get the impression that most moderately successful YouTubers make significantly more money from in-video product advertisements than they do from YouTube's ad revenue sharing.
It's also interesting how radioactive the word "influencer" has become in tech circles like HN. Nobody likes to think of their preferred internet media stars as "influencers", but they liberally apply the influencer label to internet celebrities they dislike. How often to you see Matt Levine (Money Stuff newsletter), Scott Alexander (Slate Star Codex / Astral Codex Ten), Marques Brownleee (MKBHD), or Patrick McKenzie (patio11) described as influencers? All of them are (or have in the past) used their platforms to sell products or show ads, yet they have enough rapport in the tech community that people almost never describe them as influencers. If you pretend the ads and sales pitches don't exist, or wave them away as a necessary part of their business, it's easy to pretend they aren't actually influencers playing the influencer game.
Instead, the "influencer" label has become synonymous with "fake person I don't like who shills products for money". In reality, a lot of the newsletters, YouTube channels, and blogs that the tech community loves to consume are also engaged in these very same influencer marketing circles. It's how the business works.