The primary purpose of the city is transportation. Sure, we share infrastructure like sewers and power lines, but the real reason we live in dense cities is so we can have a short travel time to the things in our lives, be they jobs, friends, shopping or anything else.
Sometimes that trip is a walking one, and indeed only the dense city allows walking trips to be short and also interesting. The rest of the trips involve some technology, from the bicycle to the car to the train. All that is about to change.