Tangentially, and anecdotally, I can see that children in Denmark (or at least Copenhagen) seem to learn more quickly than American kids to ride adult bikes because they never use training wheels. They first start on strider-type bikes that teaches them the balance needed, so once they graduate to pedal bikes they don't need the training wheel and pick it up fairly quickly - at least relative to my experience as a kid in the US.
[0] https://www.theguardian.com/cities/2016/apr/28/utrecht-cycli...
It’s one of those things immigrants need to learn when they move to this country (the Netherlands) and it’s certainly not as easy as you suggest.
There's probably a wide variance in these things.
Had I not acquired that sense of balance associated with the front bar's alignment through practice, I don't think I would have been able to hear someone say "conservation of angular momentum", pick up a bike and start riding it.
I own a recumbent bike. Learning to ride it wasn't difficult, I was able to wobble around and had to regularly put my foot down the first couple minutes and got my balance for real within a day. I let other people ride it when they ask me about it and my experience seems fairly normal.
By contrast, at one point I went 10 years without riding a bike. When I got back on, I could immediately ride. No wobbling, no learning curve.
I've seen videos of other adults learning to ride bikes and it doesn't take them long to learn how to ride but it's not immediate.