I judged seven contest entries and came to the revelation why most publishers only want agented submissions.
Out of seven contest entries, one was ready for submission, one needed just a tiny bit more polishing, three were average. The writer's were on the right track but needed to hone their craft more. Two were below average.
Now imagine if all of them were submitted to a busy editor. Yep, an agent is their buffer. If these manuscripts were submitted an agent, only two of the seven would be accepted for representation by an agent and then submitted to an editor, thus reducing the slush pile.