Interesting point, however the shipping industry is one of the least regulated in the world, hence flags of convenience, and the fact that anyone can buy a boat, and cause mayhem on the high seas, with no qualifications whatsoever. The story is a little different in aviation.
Aviation is very tightly controlled (PPL medicals for example). How many tanker captains have to undergo sim checks, route checks and medicals? No one but BA pilots can fly BA aircraft due to the nature of BA's operation, and likewise with a lot of other airlines.
I think this a good thing. Did you know that on average one merchant ship sinks every day? This goes unnoticed because they are carrying freight (not SLF!). Perhaps the shift towards air travel coincided with the slip in standards in the shipping industry, and increasing use of flags of convenience happened.
