Because GNU/Linux is the very core you need to have even a remotely functional system, i.e. you need both GNU and Linux to even have an OS of some sort, the oth

Because GNU/Linux is the very core you need to have even a remotely functional system, i.e. you need both GNU and Linux to even have an OS of some sort, the other parts are used to make the OS even more useful, but they're not the parts without which your system will fail to boot into a reasonable state to even get these other pieces working. In Windows there is lots of software not written by Microsoft, then there's the NT kernel, (which by itself does not make an OS) and the userland (which together with the kernel makes a basic OS that you can install all the other nice pieces onto to have a great experience, but the core is NT + userland)