It's called recruiting, but it doesn't work like you described.
In freelance tech recruiting, the jobs are the scarce resources, not the candidates. Freelance recruiters focus their effort on obtaining new positions for them to fill in, and only then they look for suitable candidates. You can see that in how they angle to find out about other companies that you're interviewing in - they do that, because they very much need it as another lead.
In other words, nobody would care about becoming your agent, because you're fungible. That's why it's not done in our industry. I suspect actors are much less fungible (you can't cast a 20 years old white cheery woman to play 50 years old broody black male who can ride on horses and do a british accent), hence market dynamics are different there.
I am a former recruiter and wrote an article about this years ago. One of the main issues with third-party recruiting and the recruiter/candidate relationship is that their interests aren't aligned.
In perm hire recruiting, the recruiter is typically paid a percentage of the candidate's starting salary. That means the recruiter and candidate are aligned in negotiation - if the recruiter can negotiate a higher salary, both the recruiter and the candidate are beneficiaries.
Therefore, the more the candidate makes, the more the recruiter makes. Win/win.
For contracting roles, a recruiter is a middleman and trying to maximize their margin, defined as the difference between bill rate (what the client pays the recruiter) and pay rate (what the recruiter pays the candidate).
There are only a few ways the recruiter can maximize margin.
The most obvious is to negotiate the bill rate UP and/or the pay rate DOWN. A less obvious way is to simply try to make sure the candidate with the lowest pay rate gets the job. So if I have two candidates and one is asking for 100/hr and the other is asking 120/hr, if I can get the 100/hr candidate into the job, that means an extra 40K in the recruiter's pocket for a year (based on 2000 hour work year).
I'm admittedly oversimplifying a lot of things here, but the typical recruiting model is not aligned with candidates for contract hiring.
Is there a true difference between those two models though? In the end, the client has to foot the bill, no matter how the money is allocated between commission and candidate.
As a recruiter, you could have a similar model for contractors and promise a fixed % of the daily rate. That way, the higher the rate of the contractor, the bigger the commission for the recruiter.
However, I do see your point and I notice most recruiters not being very transparant about their commission, nor do they want to give it up & work with a fixed fee instead. I know one small agency who's very open about their commission and promise to only take a cut for the first year. I hope eventually other freelancers will wise up and choose those types of recruiters over the more shady businesses.
The recruiter argument against a fixed % is that they should be rewarded for their skill in negotiation.
In other words, let's say market rate for your programming skill is 150/hr. I'm your recruiter, and I make you an offer for 170/hr. You should be ecstatic. But what if you find out that I am billing the client for 1000/hr? Would you still be happy? Probably not - you'd feel you're being robbed, even though you are being paid above the market rate.
So should YOU be rewarded for my ability to negotiate the client's rate to 6x market rate?
I'm not saying this is fair or a good way to do business, but I'm just offering you the recruiter's mindset on this.
In freelance tech recruiting, the jobs are the scarce resources, not the candidates. Freelance recruiters focus their effort on obtaining new positions for them to fill in, and only then they look for suitable candidates. You can see that in how they angle to find out about other companies that you're interviewing in - they do that, because they very much need it as another lead.
In other words, nobody would care about becoming your agent, because you're fungible. That's why it's not done in our industry. I suspect actors are much less fungible (you can't cast a 20 years old white cheery woman to play 50 years old broody black male who can ride on horses and do a british accent), hence market dynamics are different there.