Hmm, I've actually thought about this before, and I've come up with a delightfully snobby, superior conclusion. We all know that there are people who spend their free time listening to and making music and that there are people who do the same with art. These types of hobbies appeal to a specific sense; art is visually stimulating and music is an auditory delight. Then there are people who like woodworking and building/improving things.
Cars, rather than honing in on just one of these aspects, involves almost all of them (except cooking. I still haven't figured out how to work that one in.) There is so much you can do in the automotive world. Obviously, engine note is an important aspect in any enthusiast's mind, so that takes care of the auditory thrill. And just look at all the fuss E-Type Jags and Citroen DSs bring. Obviously people obsess over the visually appealing aspects of automobiles. Obviously cars are also machines that can be operated, tinkered with, modified, and improved. That's important to a bunch of people too.
Maybe you like raw POWWA! Then muscle cars/ drag racing stuff will be just perfect. Have an appreciation for the finer things in life? Nothing scratches that itch like a wafting power delivery, smooth ride, and strokable, hand build interior.
My point is, cars involve a very broad, all encompassing field of interests.