Chief Nose Wetter offers 25 reasons, including:
2. Can't get their phone to their ear with a helmet on.
6. Might have a bug hit them in the face and then they would need emergency care.
10. Their pajamas get caught on the exhaust pipes.
12. The handle bars have buttons and levers and cannot be controlled by touch-screen.
14. It's too hard to take selfies while riding.
There are many more at the link. As a former motorcyclist, I had to laugh. (On the other hand, I've seen plenty of millennials on electric scooters or e-bikes in larger cities, so maybe they've just changed the type of motorcycle they ride. Even so, there's nothing like the wind in one's face on a nice long tour of the countryside, with a good motor purring away beneath you. I remember my late 1970's BMW R100RT with great affection.)