Because, technically if you are the PNF you are not shooting the approach and not the sole manipulator of the controls. Try explaining to your next airline interviewer why you think you are justified in logging actual instrument hours when you were the PNF. You can do whatever you want but I only log instrument time when I am the FP. You can log night time ME turbine and X-C time even as a PNF because you are the required crewmwmber of a two pilot team.