The reason is simply that the OSMC settings are written as a Kodi addon, and Kodi doesn’t provide any way for us to add an addon within the standard Kodi Settings menu.
I think it is possible (I’m not a skin developer) to fake it by customising the skin, but this would only work for the OSMC skin and not for any other skin that the user might switch to, and is likely to break for future versions of Kodi and keep needing tweaking to keep it working with newer Kodi versions. (poor maintainability)
Not ideal, but until Kodi provides a standardised Skin independent way of adding addons within the Settings menu there isn’t much alternative.