Why is Mount Everest called the death zone?

Mount Everest itself isn't called the death zone. But any height above 8000 metres is called the death zone there are only 14 mountains in the zone which are: # Everest # K2 # Kanchenjunga # Lhotse # Makalu # Cho Oyu # Dhualgiri (not sure on spelling) # Manaslu # Nanga Parbat # Annurpurna # Gasherbrum I # Broad Peak # Gasherbrum II # Shisha Pangma