Actually, they are. Most places have 240 for some appliances, 120 for the majority of the appliances. 240 and 120, for obvious reasons, have different sockets. 120 has 2 and 3 prong outlets; as well as subtypes that are grounded, self grounding, tamper proof, etc.I seriously doubt that all of them are "in common use".
Much like the EU regulation on USB C, then.
Not really. If it was the same Apple would be awarded exclusive rights to sell cell phones in some countries, Samsung in others, etc. The companies would agree on standard way to interoperate for business reasons; but some pockets would only have say 3G and not work outside of their area.
The plug evolved as the result of companies wanting a safer design and the need for a standard plug for simplicity (and marketability) reasons. Standards were defined, starting in the 20’s, by NEMA, an organization similar to USB-IF in that it is a manufacturers group that sets standards. Plug building codes eventually came into play as well; but rather late in the game relative to electrification. The US can thank Edison for 110 and Westinghouse and Tesla for 60HZ; just as Europe can thank Germany and BEW for 220/50.
Unlike electrical supply, there is no compelling commercial reason for a standard phone charging plug, since the phone -cable interface is specific to a device; unlike the AC end that can be moved from location to location so a standard is needed to ensure power is available. Even teh EU recognized that customization may be warranted and thus only prescribed a bare minimum of plug/power requirements in the regulation.
Apple has been moving to USB-C for its devices and suspect the iPhone would go there whether the EU acted or not. I doubt this been speeded up the timeline since the first phone that would probably need it by reg could be as late as the 16, if Apple wanted to wait that long.