Why are aircraft windshields not unbreakable?
Windshields

In the US, aircraft windshields must conform to 14 CFR 25.775 - Windshields and windows


  (b) Windshield panes directly in front of the pilots in the normal conduct of their duties, and the supporting structures for these panes, must withstand, without penetration, the impact of a four-pound bird when the velocity of the airplane (relative to the bird along the airplane's flight path) is equal to the value of V C, at sea level, selected under § 25.335(a).


So, windshields are not indestructible partly because they are not required to be.

I guess windshields are not required to be indestructible because bodies like the FAA probably take the view that when an aircraft impacts a massive object at high speed, say a large mountain at 500 MPH, it is of no consequence whether the windshield survives intact at the bottom of a large crater of tiny aircraft fragments. It serves little purpose to make the windshield substantially stronger than the surrounding airframe.  

Glass-Ceramics

Glass Ceramics are not unbreakable. For example a glass-ceramic might have a bending strength of 340 MPa (Ref p9)