Too tricky for international flights.
Easier if you have a vast domestic flight market (US, China, etc), but not really practical if you're flying across borders, which is the base case in Europe, much of Asia, etc.
With traditional outlets you also inherit the whole legacy mess of competing standards for power mains. You don't want to feed 240V to a NEMA 1-15 outlet and melt someone's device mid-flight.
I do wonder if in some far future we'll just replace wall outlets with USBs for ordinary appliances, reserving traditional outlets for major power draws like stovetops, HVAC, industrial equipment etc. Maybe planes are the vanguard of this future?
Europe has a lowest common denominator plug, there are universal outlets (jack of all trades, master of none) that you already often find in airports, and each airline has a home country anyway.
What are you going to plug into a power outlet on an airplane that isn't dual voltage? A kettle or a toaster? I assume they have a way of preventing people from using those.
Almost all the international flights I've flown have had power outlets, always between 220V and 110V countries (heck, only Japan is 110V besides the US as far as I know).
I it works for China because they use (as an option at least) similar outlets to the USA (just ungrounded, pop).