Maybe I'm missing something about how Toptal works?
It looks to me like they bill the hiring company $X and pay the talent $Y, but the relationship with the company belongs to Toptal and the difference between $X and $Y is their (probably leaky) secret.
So looking past their 3% marketing spiel ("Google only hires the best engineers!") yes, they have a value proposition of reducing your gig-seeking anxiety, but no, they aren't your advocate the way an agent is (supposed to be). I'm not saying that's an unreasonable value proposition, but it's not what an agent does for, say, a television writer.
True, I believe they remain a middleman in the transaction. Does an agent not do that? They take a percentage fee, after all. Presumably once the contract is negotiated you work directly with the end client just like an actor would.
I believe the agent's 10% is contractually guaranteed and you deal with them whenever there's a contract to negotiate, but after that you just have to pay them.
The point is that if I have an agent, I have a relationship with my client and a relationship with my agent, both contractual and also reputational. But the agent doesn't have a contract with my client.
An agent, in the film-world sense, traditionally gets a cut of all your work while you are contracted with that agent, and -- crucially -- gets it from you. There is a contract, you're going to gross $X, you owe agent 0.1 x $X.
"All your work" because otherwise it would be too easy to make a deal with the producer, and cut out the agent. The risk on the other side is that the agent might not actually do anything to earn the 10%. You usually don't end up keeping your first agent if you are successful.
A recruiting agency gets paid on a totally different model, usually a multiplier of the recruited person's salary, sometimes tied to them staying with the company a certain amount of time. The recruiter's business relationship is with the hiring company; the agent's is with you.
So for example, for a $100,000 per year regular job, the company might pay the recruiter $30,000 after hiring you, possibly subject to your not quitting or being fired in the first three months. They are only very loosely incentivized to get you a better deal, if at all: maybe the company offers them a fixed fee, maybe they can fill the position with someone else, etc. So in a way it's closer to the moral hazard of real-estate agents.
A "proper agent" the way I'm thinking of it, for a $100,000 gig they are going to get $10,000, and if they can get you a $300,000 gig they will get $30,000. But their contract is with you -- and they get their 10% of your gross from every check, if the gig goes longer they get more money.
The incentives are quite different. In an ideal world, I would have an agent find me my jobs and negotiate my rate, because they are going to fight to get 10% of a bigger number, and me being happy and thus working more actually makes them more money. Whereas for the recruiting agency, once they get their payout they have little interest in my further success.
