Because is useless. It's an arbitrary value calibrated in BIOS by every MB manufacturer. All that matter for your CPU thermal behavior (throttle/shutdown) are core temperatures. In fact all you need to know is distance to TJMax because when your CPU reach TJMax value it will be automatically throttle itself and when it will go beyond that TJMax value (with ~30 units) it will be shutdown itself. That's all. No need for a generic CPU temperature.