It's a really obvious point if you think about it, but cars are astronomically expensive. You have to pay for them, pay for insurance, gasoline, repairs, maintenance, registration, and sometimes parking.
Many people insist on the necessity of cars. They are fast. I'll admit that. They can move you through the crudscape of suburbia like no other mode of transportation. Why linger? There's nothing much worth seeing anyway.
People are busy. They've got bills to pay. One of the most important sets of bills is transportation. And those bills are high because of all the cars we own.
In 2007 AAA calculated that the total cost per mile of driving a mid-sized sedan 15,000 miles per year is 52.5 cents ($7,875 total per year, per car). For the same period the median family income in the US was about $63,200 (before taxes). This means a one-car family with a median income would spend over 12.5% of its income on that mid-sized sedan, and a two-car family would spend over 25.0% of its income on the fleet. By contrast, a year of bus fare in Los Angeles would run you $744 per person ($62/month for a Metro monthly pass), or 4.7% of the median income for four people.
So we need to spend more of our time working, which requires fast transportation, since we've long ago given up on the reasonable idea of living close to our jobs. If cars are a "need", they're the most artificial of needs. A need we made for ourselves, and can un-make for ourselves through a little thought, effort, and better urban design.