Windshields are two safety glass layers with a plastic layer in between to hold the glass together in an accident (also helps to keep the unbelted driver/passenger inside during a crash
). If the glass is broken, it will shatter into small, non-sharp edged pieces, and the laminated plastic helps to hold together. Lamination also helps to prevent the windshield from totally shattering due to a hit from a large rock, bird, pedestrian, etc.
Plexiglass would be scratched to hell within weeks if used on a windshield. You can scratch glass, but there are places that can remove scratches. I have seen demonstrations of this while in TN.