It actually is that many vehicles don't have a steel chassis. The Model Y for instance has an aluminum chassis to save weight. There even other materials such as Carbon Fiber used in some high end vehicle.
Regardless, aluminum is expensive to extract and refine, requiring a tremendous amount of energy. In terms of cost, on average aluminum is more expensive than steel.
Huh? The main reason is because Aluminium is expensive. The BWW i3 uses an Aluminium frame exactly because it reduces weight. https://www.marklines.com/en/report_all/Munro003_201710