I don't think it has much to do with being a kid--I started with the hobby a little later than most, for one.
For me, I could give the usual "it's a challenge and requires skill bla bla bla" but that's really not it for me--plain and simple, I realize that I won't be rich and famous, and while I may own one or possibly two fun cars at a given time, I won't own ALL the fun cars I could possibly want in my lifetime. Models, for me, are a way of realizing my automotive "creativity" and building my "dream cars" without spending a fortune. I mean, literally, I'm sitting here and I can name about five different ways that I might build a '72 Monte Carlo (my dream car) in terms of engine, chassis, body, paint, etc... Would I ever be able to do that in real life? Maybe once, but not five times. So I do it with models instead.