They have a net positive effect for every owner, except that they seem to facilitate and encourage ways of living that require automobile ownership as a condition of adulthood in most places. So I'm not entirely sure they're a vast net positive in every value system. In yours, yes, but not in mine.
Automobiles are one of a key pillar of logistics. Getting things (food, medicine, construction materials, etc. etc.) to and from backbones like rail, harbors, airports etc. So even for those who don't own a vehicle or even want to own a vehicle, automobiles are still a vast net positive.
I'm not sure what the alternative would be. Maybe everyone lives in giant 10 million+ population cities that are all connected to each other by rail (and rail connects all airports, harbors, etc.) and then you have to show up at rail station to get your groceries or whatever else?
Ironically, AI facilitates self-driving cars, which promise to _reduce_ the need for private automobile ownership.