Very interesting question, and one that I would guess most people really don't have the answer to.
Sure, they can say they build models because they like "working with their hands," or because "my dad did it, so I do it too," or "I just like cars" or whatever reason you can come up with....
but I have a feeling there is a psychological reason behind it.
I once read that people who collect things (stamps, coins, dolls, matchbooks, beer steins, whatever) do so because they subconsciously crave the "control" they have over their collections. They are the ultimate lord over their collections, they control their collections, etc. Collecting is all about a psychological need for order and control in their lives.
I'm no psychologist, but my common sense tells me that we model car builders (most of us, not necessarily all of us) probably also share a common psychological driving force behind our involvement in the hobby. And I'd be willing to bet that it's about control and order. That's not necessarily a bad thing... I'm not saying that we are wackos... but I do believe that there is a psychological "something" that brought us to building models.