What they simply do not realize is that in most cases it takes 18 months to two years after signing a contract to get a book published and on the book shelves. Your correspondence with potential editors needs to reflect the fact that you understand that delay. Expecting to get your book out on the market with an impossibly short lead time will show the editor your lack of understanding of the workings of the publishing industry.
I might mention that there are exceptions to that time frame—such as with the market guide, since it is an annual publication. There are also instances where they will rush a book through the process if it is tied to a current event or happening that needs to be addressed immediately. However, it is the publisher that will make that decision.