but the key point is the contact with advertising customers, i.e., companies that want to engage in social media marketing. These contacts are only accessible to private individuals if they already have one or multiple successful accounts, which unfortunately only very few of those aspiring to a professional career in this field ever achieve.

I get the impression that you also generally have to already have a successful account to be considered by agencies, which would defeat the point somewhat of it being a way to get over the initial hurdle. I watch vtubers on Twitch and from what they sometimes say about how sponsorships work, much of it is somewhat automated and gated mainly by account popularity metrics, which makes sense because why would advertisers want to pay a premium to another middleman if they didn't have to? There was a vtuber agency that collapsed recently when it came out that they were insolvent and had been defrauding many people they worked with along with various other corruption and abuse, and given how similar scandals aren't uncommon and the need for creators to be doing the work of building themselves up as a business regardless, makes it seem like a pretty bad deal to have an all inclusive sort of contract with agencies.