To prevent the urethane adhesive from breaking down under sunlight, the glass is coloured black around the edges. The glass has also become thinner. Compare a windshield from twenty years ago to a new one today and the new one will be about half as thick and weight. In the quest to lighten the automobiles and reduce vehicle weight for improved fuel economy, glass has been changed too, but the method of bonding the glass in place has made the assembly stronger than ever. Finally, windshields are expensive.
Most automotive glass comes from only a few manufacturers.
The windshields with the best optical clarity are used for HUD systems. The next best quality ones are used by the automobile manufacturers for new car production and replacement glass sold at dealerships. The rest of the glass goes to the aftermarket glass shops. There may be slight flaws so the glass is cheaper, but if the flaws are not obvious, then the glass is a good deal. Last, come the rejects.