As impressive feats of small-team engineering, the X-Prize finalists are amazing. But they are nothing like consumer products, just as NASCAR "stock cars" are nothing like consumer products. The X-Prize competitors were given a set of design constraints, which to Progressive's credit did include basic performance and safety criteria, and optimized their designs to meet those criteria. In other words, they built cars that were going to have very high efficiency and a bare minimum of performance.
If you were to take an X-Prize winner, and try to make a mass market product out of it, that would meet FMVSS standards for a passenger car, 150,000 mile reliabilty, NHTSA's crash standards, would be comfortable and convenient, easy to drive, and affordable to a middle class family, you would not wind up any where close to 100 MPGe.
You would probably wind up with something like a Nissan Leaf or Chevrolet Volt. But without the economies of scale in purchasing, design, and development that a major OEM has, your X-Prize car would cost thousands of dollars more.
The OEMs could have easily competed in the X-Prize competition, if they wanted to. They were too busy making vehicles people would actually consider buying.