Why are shaver outlets standardised worldwide, but regular electrical outlets aren't?
That's not the half of it.  Go in any post-1940 sleeper car at a railway museum.  There's a shaver outlet and guess what.  Mind you, there are only two power sources in the car, a 32v battery charged by wheel driven generators... And service steam.  I'm not sure if they tap the generator to get something vaguely resembling 110VAC at random frequencies, but this passenger-car power was  exceedingly filthy and only good enough to run a shaver, and the receptacle is placarded "shaver only".  

Amtrak's new cars retained this placard needlessly, when they standardized train power at 480V delta served from a generator car or inverter on the locomotive. 

At the time this became the norm on rail, trains were THE way to travel. I'm sure passenger liners had similar receptacles, and this spread all over the world. 

"110V-ish good enough for a small brush driven DC style shunt motor" is not a particularly high standard to hit.  Frequency is not an issue, and a simple transformer will do the job.   

Frankly given the ubiquity of digital switching power supplies, I'd expect a new world receptacle standard which promises only some random voltage between 74 and 277 at some fixed number of watts. 

As for sub-10-watt receptacles, there's already a standard for that.  USB.