People who are on a low income must spend a disproportionate amount of that income on owning a car, which is a necessity since most jobs are inaccessible without a car. Those lucky enough to have access to transit are severely limited in their mobility, and therefore have a much more limited range of employment options. In addition, suburban areas offer very few housing options for people on low incomes. The result is that poorer sections of society become concentrated in trailer parks, public housing projects and areas of dilapidated housing. All of a sudden, you can't visit relatives, you can't so grocery shopping, and you can't get to the golf course, church, library or wherever else you like to spend your free time.
School is only accessible by excruciatingly slow buses.
Football practice, parties, dates, and most of your friends are only accessible by asking parents for rides. As an eight year old boy in London, I was allowed to walk to the corner shop to buy sweets, comics, and whatever else my pocket money could afford. By age 11, I had gained enough proficiency to ride my bike on the road instead of the sidewalk, and I could access the park, the library, the swimming pool and a whole range of shops. At 14, my parents allowed me to take the train to central London on my own, to spend the money I earned delivering newspapers. By the time most American teenagers get their first glimpse of freedom at age 16, I was completely independent.