I just watched a TikTok about how people used to plug in things to their light sockets when electricity was first becoming popular. And they kept calling the plug a light socket, what rule says it’s not a plug that screws in? And why shouldn’t plugs screw in? We have material science to make plugs and cables hard enough that if you kick the wire it still doesn’t come out. Electrical connections should be more secure. That’s all I’m saying.
One reason would be so that if you needed to unplug something quickly, you can do so. Also you wouldn’t be able to have so many different variations to differentiate the different amperage and voltages. Most residential dwellings (in U.S.) only use 15A or 20A 120V receptacles, but there are plenty of other different types that use horizontal or verticle or even anhled slots. Think of a 240V air conditioner, it has a different type of plug so you don’t accidentaly plug it into a 120V receptacles. If needing something more secure, you can always get a twist-lock plug.