I think you have the question backwards. I wonder why more people, especially in tech, don't have agents.

Broadly speaking, writers are freelancers who enter into short-term high-value contracts for work-for-hire. Agents negotiate the contract on the writer's behalf, with the agents compensation depending on the writer's (this, in fact, is what the current dispute is about, as this has become less and less true). This ensures that the writer gets a good deal, ideally the best deal possible. Talented writers aren't necessarily talented negotiators or networks, so this division of labor works well.

Seems to me everything I just said also applies to freelance Tech Workers. But instead of Agents we get marketplaces like oDesk or TopTal where freelancers are competing against each other, often in a race to the bottom.