Why does the internal weather filter inside some of your vents feel somewhat flimsy?

The internal weather filter has to be as low density as possible so as to not restrict airflow through the vent.

The internal weather filter has to be as low density as possible so as to not restrict airflow  through the vent. The denser the fiber/content, the higher the resistive pressure through the filter there is for air to flow. As a result, you would need higher winds to exhaust the same amount of air as you would need with a lower density filter.