The system is designed with the Intel GPU being the sole one connected to the internal or external display panels. The nVidia GPU isn't -- and cannot be made -- a primary GPU; it is solely a co-processor for the Intel GPU. It is controlled entirely by the video drivers.

There are systems where there's a hardware (or firmware) switch that allows the selection of the GPU at that level -- the new XPS 17 with the 2000-level GPUs is designed this way, as are many Alienware and other gaming systems, and some Precision mobile workstations. The XPS system you have is not engineered this way - it is entirely software controlled, and only the Intel GPU connects to the display devices.

The discrete GPU is not disabled other than when you do so by software settings, but neither can it directly send data to your display screen -- it must channel its output through the on-CPU Intel GPU.