The nib has dried out even though I was closing it every time after using the marker.

Since the cap is made of plastic, it may crack or deform when exposed to strong external pressure, making it impossible to maintain airtightness. Also, if you leave it in a place with high temperature (e.g. right where the warm air from the heater hits) for a long time, even if the cap is closed, the ink inside may dry out and you may not be able to use it.