Why can't OS use entire 64-bits for addressing? Why only the 48-bits?

Question

I\'m reading \"Understanding Linux Kernel\".

Paging for 64-bit Architectures

As we have seen in the previous sections,

Answer 1

Its just hard to believe for me that 32-bit addressing ie.. 4GB (2GB/3GB to be more correct) address space per process is a limit. If you really encountered this limit. Please give me example.

It doesn't exist any more (except on some old employees personal machines) but I worked on a suite of software called RealiMation back in the late 1990s/early 2000s. It was a real time 3D engine for visualisation and simulation. One of our customers regularly created highly detailed models that hit the 2GB memory limit. We would load textures on the fly as and when needed and had to add code to check for memory allocation failure so we could continue displaying the model, albeit untextured.