Here's why:

* Suberin: Cork cells are characterized by a thick, waxy substance called suberin in their cell walls. This suberin makes the cells waterproof and impermeable.

* Cell Death: As cork cells mature, they actually die. The cytoplasm and other internal components break down.

* Air-Filled Spaces: The space within the cell, once occupied by the living contents, becomes filled with air. This air is what gives cork its characteristic buoyancy and insulating properties.

So, while technically empty of living content, cork cells are not completely empty, but instead filled with air.