In what universe should the agency get 70% while the person doing the work gets 30%?

Yeah, 5-15% sounds awesome. I was recently considering doing some project-based work through a consulting firm and their cut was more like 60%. Wouldn't have to do any sales, but that seems absurd.

I wonder if it is a US thing of employees having too little power. A couple years ago I heard an interview on Fresh Air [1] where a woman talked about her experience cleaning houses. She said she at first made $8.55/hour while the agency charged clients $20/hour (later it was $9.25 and $25, respectively). That means the agency got 57% (63%). I told my wife if we ever hire someone to clean our house I'm asking how much the person doing the work gets paid, and if it's not above something like 70% of the total, we're not doing it.

[1] https://www.npr.org/transcripts/689611873

In Europe it's very usual to charge consulting rates that are double what the employee is paid. After the employer-paid portion of taxes and insurance it's necessary, the actual margin is just around 15-20% then.