I always thought it was because they were considered "double insulated". Here's an excerpt for the definition
Double insulated or class 2 electrical appliances are products that have been designed in a way so as not to require a safety connection to electrical earth (These products must NOT have a safety connection to Earth).
These products are required to prevent any failure from resulting in dangerous voltage levels becoming exposed causing a shock etc. This must be done without the aid of an earthed metal casing. Ways of achieving this include double layers of insulating material or reinforced insulation protecting any live parts of the fitting.