cars are filled with compromises. long narrow space would only fit a very small circular speaker in many cases.
as said, the oval is to fit a space, sacrificing accuracy for some more bass which helps when theres road noise to overcome. most cars are rather lousy acoustic environments. the noise floor of the road/tires/wind mean that you have to put out that much more just to be able to hear any bass let alone the other frequencies. never mind the enclosures/mounts that aren't very stiff, bouncing sound off glass etc.
bestbuy has ok bookshelf speakers for a decent price. think there were threads on either avs or head-fi about them. insignia i think.