There are a couple of other considerations. Starting any hobby is hard. There are a lot of skills that have to be mastered and at each level frustration can set in that makes the person give up the hobby. There is a reason we see a lot of lightly used sporting goods at garage or yard sales. Model building is no exception. It can take 5 to perhaps 10 models before the person sees real improvement. There is the monetary cost, but there is also the fact that frustration may set in and the person does not want to stay with the hobby.
The other point that I think is important is that the time a lot of kids have is highly regulated. They go to school and then are whisked away to an activity after school. Then they go home to eat supper and then either have to do their homework or they may be whisked away to another activity. My kids are in their 20's and we did this to a certain extent, but I hear some parents talk about how they run their kids around, and the kids don't have the time to start many other activities. The Playstation or the Xbox are easy diversions. The games cost about $60.00. They could have hundreds of fails learning the game, but the cost is still $60.00.
Bluenote made a brilliant point about the sheer number of other options there are available to kids, but they are also available to adults. Times are vastly different now and there is so much more that can draw a person's interest.