Young adults are soft, poor, spoiled and want to carry stuff (surfboards, skis, drum sets,...). Motorbikes don't fit their lifestyles.
My teenager expressed interest in riding. So, I offered that he could take the safety course and ride an underpowered trail bike (say, a TW200) to high school for a year, then I'd think about letting him ride a proper motorcycle. He wanted nothing to do with it. Too wet, too cold, not cool enough...
'74/'70 R98/6/GS Traveling Bike (construction under way)
'91 Bill Holland Steel w/Dura Ace