It makes a bit of sense in the context of the latter two; the contracts are worth so much money that an agent can make a living off of a small percentage of the take. But if you're a programmer who can't handle your own finances, PR or salary negotiation, how well-rounded are you?
There are more programmers than athletes and Hollywood stars, but there are many more people in industries where using a recruiter/agent is not the norm. If you think that programmers are good at or enjoy handling their own finances, doing PR, or salary negotiation, I'd encourage you to talk to thousands of programmers and ask them how they feel about that. I have programmers come to me for advice on negotiation and what career moves to take that aren't even active candidates (jobs they found on their own, yet still seeking representation). Programmers that I have met hate that part of the business, hate negotiation and building their brand. Of course there are some that are willing to do it and are good at it, but the majority seem to be completely fine passing that off on to somebody else if they can.
What you say is reasonable.
I'm not sure how I'd answer that question, however, which I guess brings us full circle to why it seems frothy to me. I'm not arguing that there is no business to be had in the space; apparently, it's happening.
But the fact that there is actually demand for such an occupation to require or request agency representation is so far from the norm, it seems like an anomaly, destined to be corrected. Of course, I'm using hindsight, not foresight.