Uh, possibly because one doesn't have an iPod, or really want to shell out for one?

or possibly that those instructions don't work for all years or models of the factory stereos. I know for a fact the 2003 RF system does not have an aux input.

Personally I have no use for a $250-500 mp3 player when I can have one that's always in my truck ready to go. Buying a iPod, accessories seems like overkill when I have one that costs less and sounds better thanks to the stereo upgrade.