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.
The rear underbody of the Y is aluminum but it still has a steel chassis. Aluminum can never fully replace steel because it lacks the strength and rigidity.
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.
None of what you said contradicts what you quoted. Steel is a stronger material than aluminum, the trade offs are that it's harder to form and weights more.
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.