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Inertia, note that NEMA was developed in the early 1910s by Harvey Hubbell, who was a businessman — hence the socket is optimized to be cheap. Type F by Albert Büttner was developed more than a decade later in the mid 1920s.

https://illumin.usc.edu/a-powerful-history-the-modern-electrical-outlet/ https://www.plugsocketmuseum.nl/NorthAm3.html

Actually Type F is not that great as a plug either. It is also one of the older, overly bulky design, and predates polarity. And the shape allows you to cheat by inserting into an unearthed receptacle (e.g. CEE 7/1), and the lack of polarity makes the ground pads a shock hazard. Even with an earthed CEE 7/3 receptacle, the live pins are in contact first, while the ground pads still are touchable. There is also the additional annoyance that even within Europe/CEE 7 there is the competing and polarized Type E, necessitating that virtually all modern appliances come with an overly complex CEE 7/7.

The Swiss have developed Type J or SN 441011, which is a modern design far superior to Type F. The internationally standardized, but shape-incompatible version is Type N or IEC 60906-1, which is adopted in Brazil and South Africa.