Why did they design a car that would go over a thousand miles on a tank?

I would have thought that, if nothing else, a ten-gallon tank would be unnecessarily heavy, but I do not think that I appreciate how small gas tanks actually are. I found an aluminum five-gallon one on eBay that is 12"x12"x8.5" and 8.25 pounds. Their ten-gallon one weighs 12.5 pounds.
Why are you wasting 4.25 pound?!
