I think being preoccupied with parts of object is a symptom of being preoccupied with objects in general.
When an NT child plays with a toy car, he is not interested in the toy car object in itself. Rather, he is interested in it only as a representation to real life cars, and the associated experience of the thrill of driving which excites his young imagination.
On the other hand, I suggest that when an ASD child plays with a toy car, he sees it simply as an object in itself. It is free from any representations to real life things. Therefore, he will do odd things when exploring the car, e.g. spinning its wheels. Since he does not see its link to real life cars, he is less likely to do the standard thing to it (drive it around and make vrooom vrooom noises)
So in reality, IMO its not really that ASD children are obsessed with parts of objects as such. Its that instead, NT children aren't preoccupied with objects at all. Rather, they are interested in the experiences those objects represent to them.