Because gravity points down. Spheres are optimal ways of dealing with things that are equal in all directions. Cars aren't equal in all directions. They have a top and a bottom, because the bottom part has different concerns (contact with the road) from the top part (keeping out the wind and the sun, keeping you from flying out). It has a front and a back because its operator has eyes that point forward, and it only moves forward. A sphere is not aerodynamic.
Making it roll would be unstable; it would wobble to left and right. And it would distress the bejeezus out of its occupants, who aren't up to tumbling (especially not side to side). They'd be unable to see in the direction of travel at least part of the time unless the entire thing were transparent, and there are no good materials for that.
A sphere is a good way to enclose a volume at minimal surface area, but there's no need to optimize cars that way: the surface metal is not the expensive part. And a sphere is difficult to pack: much of the space is wasted unless you're filling it with liquids.
So overall, there's no good reason and many reasons not to.
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