It’s simpler than that. Developers have a self-reliance complex in the extreme, and the prevailing attitude is that anyone good doesn’t need a talent agent.

Also, there are many who’ve tried to establish themselves as talent agents: some ex-recruiters, some ex-talent agents from the entertainment business. I’ve followed them and none of them seem to be any good. Employers also believe (at least, for now) that any good engineer can negotiate pro se. It would take a lot to change this culture.