IMO it is not so much about the States or the Feds getting more $$ out of these licensing schemes but mostly the insurance
companies having another excuse not to pay a claim. "You did what? Let your 16 year old unlicensed kid touch the steering wheel
while under way? What do you mean how we know? It's right here on facebook! No claim pay out for you, Mr. Boatie." Just follow the money
and ye shall know who is pushing this crappy idea.
Comparison to MDs is inapplicable as the number of residency slots is highly controlled by AMA and Fed gov't. Thus not all of the potential residents, i.e. those who passed all the pre-requisite tests are admitted to residency and thus not as many licensed MDs are churned out as would be in an open slot system. But even that said I don't think licensing MDs is a panacea from stupid, lazy or just plain ignorant as evidenced by the stats of "death by MD mistake" being 3rd most prevalent type of death in the US.
If anyone has an itch to license
some human activity or other let them satisfy it by clamoring for licensing of baby births or some such. Much more long term downside potential in that activity for the society at large than from some ignorant boaters.