People in their 20s - 40s (I say as a 50 year-old) are quite busy starting careers, finding mates, starting and raising families, etc.
I was a wargamer in my teens, completely stopped when I went off to college, and only in the last 10 years have I resumed it. As my kids have grown up and gained independence - giving me more free time - , and as my net worth has modestly increased, I am much more active than I was wargaming in the past.
My son's story is analogous. He is 18 years old. For the past three years, he has been very, very active in astrophotography - spending thousands of dollars and making beautiful photographs of the stars. We have taken vacations based around where he can go for dark skies.
But he just started college this year. He didn't take his astrophotography gear with him. He is concentrating on challenging engineering classes, his private pilots license, and his newfound hobby - triathlons. I expect those to be the focus of his life for the foreseeable future as he pursue grad school and a career - and who knows what else. I could certainly see him re-engaging in the astronomy hobby (the local astronomy club is full of grey-haired retirees who have time for the hobby) sometime in the future, because he still loves it, but as a young adult who isn't a trust fund kid, the demands on his time and energy are too much for him to pursue some of his passions.