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.
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.