I asked myself the same thing about wrestling a long time ago. I'd stubbornly watch an hour here or there asking myself; why... why! And then I had an epiphany. Sports are just soap operas for men. It's about the personalities and relationships more than anything.
NASCAR is like that, where the relationship between drivers is played out in front of the audience in the form of race strategy. There's a lot of southern social norms that are used to judge drivers. Loyalty, standing up for yourself, paying your dues, respect for elders, being genuine, spirtuality... in the eyes of fans, these codes of behaviour all manifest themselves during the course of a race and the fans relate to the fate of the drivers according to how they fall in line with their values.
The seemingly slow, droning repetitive motion of the cars circling the track is a metaphor for the fan's lives. It's refelects the pattern of their work, family, leisure time, and the relationships that slowly unfold therein. This is how they relate, how almost every sports fan relates to their sport.
At the same time there is always the car side of things to appreciate. These are very sophisticated machines within the restrictions of the rulebook. And as has been said already, TV doesn't do justice to the speed, noise, power of the live show. These are the more immediately appreciable aspects of the sport that initially attract fans, until they start to identify with the interpersonal, 'soap opera' side of the sport, which will keep them interested in the long terms, usually for at least one driver's generation.