The answer to 'why is my dryer vent filthy' is that a buildup of lint and debris in the dryer vent is a common issue that can cause the vent to become clogged and dirty over time. This happens as lint and other small fibers from your clothes get pushed through the vent during the drying cycle and accumulate inside the vent over time.
Some common symptoms of a dirty dryer vent include longer drying times, the dryer running hotter than normal, and increased energy bills. To solve this problem, you'll need to clean out the vent on a regular basis, either by doing it yourself or hiring a professional dryer vent cleaning service.