Motorcycles and aviation catered to a certain demographic of people looking to get out there and do something interesting, something crazy. Perhaps they were the adrenaline junkies of their time.
I have a bike license. Haven't owned or driven a bike in ten years, and by now I probably never will again.
Part of it is simply growing up. It's just not as much fun any longer as it was in my 20's. And with work and other committments I have little time left to ride, never mind maintenance and other chores.
Why young people don't ride, though, has - I guess - nothing to do with risk. Driving is simply not fashionable, and not cool. A generation earlier than me, getting your license (and a bike license especially) was a badge of honour, and a symbol of adulthood. It isn't any longer. Many hobbies rise, flourish and die over time. Once, wood lathing was a major, very popular hobby across Europe and the US; today it's a tiny niche. That's probably where these things are heading as well.
Car ownership has suffered the same decline in cool, but as cars are utilitarian they don't see as large a drop in usage. Young people still drive, but see cars more akin to owning a washer and dryer, not a status symbol. Necessary but boring. Bikes and private airplanes don't have the same level of utility in general, so they suffer more when interest wanes.