That's what many people think, but it's actually not true (at least not in the states I've worked in). There's a cottage industry that provides advice on how to navigate through the RFP process, and a lot of the advice is based on controlling what goes into the RFP when it is written. Take a look at this set of slides:

http://www.slideshare.net/pmcloughlin1/how-to-help-your-cust...

The most important thing to understand about RFPs is that they aren't designed for making the "most fair" selection. They are designed for making the "safest" selection. An RFP is basically a CYA (cover your ass) mechanism for the procurement team. That's why collaborating with preferred vendors when the RFP is being written is a very common practice. Teams writing the RFPs rarely have the subject matter expertise necessary. So they receive help (which is a good thing).