The answer to 'why is my dryer vent greasy' is that a greasy dryer vent is usually caused by a buildup of lint and debris in the vent. As hot air and moisture from the dryer pass through the vent, the lint and debris can collect and become coated with grease and oil from the clothes.
This grease and lint buildup can restrict airflow, leading to longer drying times, increased energy usage, and even a fire hazard. It's important to clean the dryer vent regularly to prevent this issue.