Here's why:

* Stratus clouds are low-lying clouds that appear as a gray, featureless sheet.

* Fog is essentially a stratus cloud that forms at or very near the ground. The difference is primarily in location and how it forms. Fog typically forms when air near the ground cools to its dew point, causing water vapor to condense into tiny water droplets.

So, while fog is technically a type of stratus cloud, we give it its own name because of its unique formation and location.