The problem is Qubes is not an ordinary operating system. The Qubes developers are not writing the whole OS but reusing a lot of things from others. For example, Xen is used as the hypervisor, Debian and Fedora are used as the operating systems in the VMs by default, and Fedora 32 is currently used in dom0 for managing the VMs and GUI.
The latter defines how your GUI behaves. For example, hitting Alt+F11 makes a window go full screen and hitting Alt+F3 gives you a menu for choosing which app to run. You cannot possibly put all Fedora, Debian, Xen documentations into Documentation | Qubes OS, this would not make it more readable. So for non-Qubes-specific questions users should go to the docs of the corresponding distribution. Perhaps, the above text could be added to the FAQ (which is already very long).
I am happy that you also think this way. I agree with you.
I said that there are several discussions on this topic and only linked one. You could use the search on this forum to find more discussions. I guess this one should be more helpful: Which one is more secure: "sys-usb" or "sys-net as sys-usb"?. In short, all your usb devices will have the access to the Internet. Doesn’t it make you less secure? See also Device handling security | Qubes OS.