All flight simulators that I know of have a visibility slider, except MSFS. The ability to set an accurate visibility and RVR at any airport without having to rely on a METAR is a must for IFR training.
If you want to practice low visibility approaches, finding an airport somewhere around the world where there happens to be dense fog is not a suitable workaround, especially if you need to practice the approach at a certain airport. Adding custom weather presets is not a suitable workaround either (too time consuming, not user friendly).
Note that this is not a discussion whether or not fog is possible in MSFS. Fog is possible but it is not possible to set accurate fog at an airport via the Weather Settings dialog. You can read more about that topic here:
What we need is a slider in the Weather Settings dialog to set the Visibility and RVR.