Great post.
I think I can answer this question, at least what seems to be the reason for me.
As an adult we have too many things to think about, to worry about. We have so many responsibilities, and we just flat out don't have the time to play.
When we were younger we had not experienced as much of life as we have now. Life was simple, you thought of nothing but what am I going to do right now, lets play for a few hours. now it's crap, my car payment is due, I need to water the lawn, has the dog been fed? Did I pay the Credit Card, I'm late for work, Will I be able to get that proposal done in time.
Adults have more important things to think about and do (unfortunetaly).
I too remember playing for hours and when I recently picked up all my old LEGO's from my parents house, alot of my old creations were still in tact, and it almost brought a tear to my eye, remember the exact way I built a certain set.
Well, for me collectiong is a way to hold on to what I had as a child, and to me, bulding is playing. I usually build while watching TV so it doesn't appear to be a complete waste of time to my wife.
We just have different priorities now.