The Federal Motor Vehicle Safety Standards have a lot to say on the strength, composition, and properties of glass in passenger vehicles. In addition to being very difficult to break, it must also be resistant to shattering and remain in place while experiencing some pretty extreme forces.
Automotive glass is not even glass really, it's more of a composite made up of layers of various transparent materials all designed to counter balance the weaknesses of the other layers. And windshields are bonded into the body of the vehicle with a very strong plastic adhesive.