Because it would counterproductive to do relative to your monitor display, so you have to have perceptual from your image file to the monitor. You will see whatever the image is in perceptual without further clipping. But you have to remember that you can't see anything but relative when you convert your working space from say argb to srgb. You can also get perceptual by soft proofing or by using Photoshop manages colors and selecting your printer's perceptual. If you take raw and develop to ProPhoto, you will see on the monitor perceptual in the monitor's space, hopefully argb if your monitor is wide gamut, so that's not too bad. If you convert the ProPhoto to srgb, you get relative. There are a couple of ways to get perceptual when doing that but it involves finding either the ICC v4 profile and installing it, or getting the other conversion profile from a member of this forum whose username I don't recall at the moment.