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In addition to what GGx and robertcday have mentioned, it is because the submissions you are talking about, the over the transom submissions, are their lowest priority. Publishing houses, agencies,...
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Source: https://writers.stackexchange.com/a/37316 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/37316 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
In addition to what GGx and robertcday have mentioned, it is because the submissions you are talking about, the over the transom submissions, are their lowest priority. Publishing houses, agencies, and magazines would much rather deal with people they know, people referred by people they know, people they have met at conferences and struck up a rapport with, people, in short, with a higher probability of producing publishable stuff. If they could generate enough content from just these sources, that is what they would do (and many of the larger publishing houses and magazines can generate more than enough by these methods and have therefore nailed the transom shut). Reading the stuff that comes in over the transom is like fishing in the Sahara, so they only do it when their time cannot be spent in any more useful way. And only publishers at the low end of the totem pole who can't get all the usable content they need any other way, will do it at all. So if you are unknown as a writer, and don't have a relationship with anyone in the industry, you are stuck waiting for someone to have time to read the slush pile. And it can be a long wait. The wonder is not that most markets take a long time to respond, therefore, but that there are any that respond quickly.