It is fed by search committees who don't know what they're searching for and consultants who need to justify their fees by creating monumental RFPs filled with grotesque irrelevancies.
As someone who has participated in a thousand agency searches, here's how I would do it.
Dear XYZ Agency,If the answers to these two questions don't tell you everything you need to know about the agency, there's nothing wrong with the agency. There's something wrong with you.
We are looking for an ad agency. We would like to consider yours. If you're interested, please read the information we've attached, then answer two questions:
1. In your opinion, what are the key marketing issues we're facing?
2. If you were our agency, what would you do about it?
You may take as much or as little time as you need. Please call me and let me know when you'd like to come in to answer these questions.