Because the North American two-pin plug (Type A) was originally not "polarized".Why would they want a polarized two pin plug?
So, the NEMA 1-15 Outlet was modified so that the Neutral Pin was Wider than the Line Pin.
UK socket-Outlets (Type G) are not intrinsically "polarized".
Heever, they are polarized by default, since no two-pin plug can be inserted.
It is necessary that a three-pin plug must be inserted, so that the Earth Pin may operate the shutters.
Australian 3-pin socket-outlets (Type I) are intrinsically "polarized".
In most Australian socket-outlets the Earth Pin is not required to open any shutters.
Hence, Polarized 2-pin plugs are available and used for "Double Insulated" devices in Australia.
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