They should but the reason it will never happen is political. Just look at the comments here. Dozens of cyclists throwing a fit because you have the gall to suggest someone operating a vehicle on a public road might be subject to licensure. They'd never vote for a politician supporting that, and they'd work to defeat any who did. On the flip side, the folks who agree with this are not going to vote for a politician they otherwise wouldn't simply because they suppose bicyclist licensure.
It's the same reason why you need a dog license but not a cat license in many areas of the US. The cat people will freak out when you suggest a $10 license and nobody else gives a shit.
Ahh.... It's actually happened. Various governments have promised to introduce licensing schemes. I don't think there's any current ones around. New South Wales had a plan, but they abandoned it just before implementing it. Not because of the voting power of the lycra lobby, but because it's bad public policy.
As for how good an idea it is to have a licensing system for people travelling without lethal weapons, papers please! It's a fantastic idea. You probably also should have a chip in you so the government can track you and see if you've travelled to a part of town you don't have a licence to be in. After all, we've got to cut down on crime.
It's the same reason why you need a dog license but not a cat license in many areas of the US. The cat people will freak out when you suggest a $10 license and nobody else gives a shit.