The answer to 'why is my dryer vent grimy' is that a grimy dryer vent is typically caused by lint buildup over time. As your dryer operates, small fabric fibers (lint) are produced and pushed out through the vent. Over time, this lint can accumulate and create a grimy, clogged vent.
Some common symptoms of a grimy dryer vent include longer drying times, the dryer running hotter than normal, or even a burning smell. If left uncleaned, a grimy vent can become a fire hazard and reduce the efficiency of your dryer.