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Because evaluating an author's work is expensive. It consumes time that could otherwise be spent finding other authors. That time is a dead loss if the author signs with another agent. In the end ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/20768 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/20768 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Because evaluating an author's work is expensive. It consumes time that could otherwise be spent finding other authors. That time is a dead loss if the author signs with another agent. In the end this is a matter of power. While the agent does work for that author, there are more authors looking for agents than there are agents looking for authors, so the agent gets to set the terms of the courtship. If the author comes with an existing platform, of course, then the power is reversed. If George Clooney wants a literary agent, he can submit to as many as he wants and they will drop everything they are doing to read his stuff. (If they even bother to read it.)