<50cc scooters are subject to different -- usually lower -- registration, insurance, inspection, and licensing requirements than cars.
State-minimum insurance for my cheap (KBB ~$1000) used (+200K miles) car would be about $700/year. Similarly minimal insurance for a 2010 Honda Ruckus would be $100/year. (I just checked Geico.)
Small motorcycles consume a lot less gasoline than cars, especially old, cheap ones being driven in-city.
Spare parts for scooters can be purchased and shipped cheaply, and assembled in a minimum of space with cheap hand tools.
Scooters don't require a full parking space to store. They can be stashed against signs, at bike racks, next to doors, in tiny alleys, or any other free spot. Living with other people? You don't need to worry about blocking the driveway, or being blocked yourself. Work downtown? You can get away without buying a pass at a parking garage.