Plus, as an employee, I negotiate salary and benefits with confidence but always keeping in mind that I'm negotiating against someone who I have to work with on

Plus, as an employee, I negotiate salary and benefits with confidence but always keeping in mind that I'm negotiating against someone who I have to work with once we sign the contracts - you can only push your position so strongly while maintaining a comfortable working relationship for the long term, you can say "its just business" all you want but people on the other side still get their feathers ruffled. In some sense, the agent or recruiter here is also being paid to be the asshole - everyone expects it of them, hardball is part of the deal, the employee being hired can just sit back and shrug and say "hey, my agent is tough to deal with sometimes, but they're good at their job" and reduce confrontation with their new boss. On top of that the agent has perspective on market rate for a role that goes well beyond what you could hope to gather in Glassdoor, by asking friends, etc. If it were commonplace in tech, I'd absolutely hire an agent on my behalf if I were doing gig work.