I am sure it's not hard.
If you learn to fly, from a mate of yours, and know the protocol (ground and radio) you can fly everywhere without any papers, and certainly without ever having to do any maintenance on the plane apart from (as on a cheap car) changing the oil periodically.
It's been done, too...
There are plenty of keen simmers who know how to start up and fly a Citation, and how to handle ATC (VATSIM etc). Combine that with a bit of hands-on, and off you go
I've done the JAA and FAA PPLs, the IMCR, the FAA CPL, the JAA and FAA IRs, and the portion of all that that I actually use on a flight across Europe is about 1%. Most of what's needed is operational knowledge (protocol) and some aircraft performance if crossing high ground, but a Citation is not short of perf
The flight training business is set up mostly upside down (training mostly stuff you don't need to know) to keep a load of old (mostly non flying) fogies in their salaries and pensions and to keep up certain barriers to entry.