RFPs are issued by firms that have a limited understanding of the item that they are seeking to procure. The RFP provides an issuing firm detailed data from outsourcing vendors on how not just the cost of labor, but also on other key aspects of outsourcing: cost of facilities, availability of telecommunications, local risks (weather, politics, military threats), etc.
RFPs are usually issued by a procurement department. Outsourcing is one of the most sophisticated products that a procurement department can source. It may take months or more to develop an RFP for a very large outsourcing program.