Why? Partly because many young(er) people don't actually see flying the average spam can as particularly exciting or adventurous
Then that's partly the fault of those of us who are established in the pastime. There is one certainty about flying: Either you are, or you are not. Once you are flying, there are thousands of different ways to do it. Each has its merits and not so attractive aspects. WE have to sell the merits of each kind of flying, without letting the less attractive aspects seem to prevail.
There's no point in complaining that you can't take the door off the G1000 C182S to hang out taking photos at 60 MPH on a calm night. And worrying about why a Tiger Moth is a poor IFR platform is kind of pointless. The Twin Comanche is a poor choice for aerobatics, but then loading up the Pitts with a week's baggage, for a holiday to off shore islands may not be a good choice either.
Happily, I still see young people out on bicycles (often when I'm out on mine). They are not driving their Jaguar or Range Rover (that day, anyway). So as a bicycle will provide predictable transportation and fun, so will a simple airplane - it is up to us, those who really appreciate simple airplanes to sell them to the next generation of pilots - because they
are flying.