I know that cars are made of steel but why arnt they made of aliminuim like planes?

Because aluminium costs significantly more than steel.

I believe that some jaguars have an aluminum body. Aluminum is too expensive!!

no one expects to survive a plane wreck
a steel plane fare would sky rocket
but a car wreck'd better a survivor make

Aluminum is too expensive, and weight matters a lot less than it does on an airplane. You'll find aluminum in higher-end cars, and often in your engine block, but usually the body is steel for mostly economic reasons.

Aluminum is on a strength per strength basis is about 1/2 the weight and twice the cost as steel.

Also, steel is much more resilient than aluminum. With a typical force that an auto chassis may typically face most aluminum features would deform.

You can use aluminum in many applications for cars. They have to be cases where the shape wont change much due to having an ability to design the parts to be really robust.

Aluminum wheels are easy to think about because there's a definate performance enhancement because autos will have less 'unsprung weight' on the shock absorbers and springs. There would also be less wear and tear on the clutch compared to steel wheels