The answer to 'why is my air duct greasy' is that air ducts can become greasy and dirty over time due to a buildup of dust, debris, and grease from cooking and other household activities. This can happen even if you clean your home regularly.
Some common causes of greasy air ducts include cooking oils and smoke circulating through the ductwork, pet dander and hair, and a lack of regular professional duct cleaning. Symptoms of greasy ducts may include visible grease buildup, musty odors, and reduced airflow from your vents.