Polycarbonate has one of the lowest ABBE values of any of the optically used plastics, so it scatters light pretty badly the thicker it is. As compared to eyeglasses, telescope lenses are really thick, so the poor light transmission would ruin the clarity. On top of that, poly expands a lot under heat, and so any coatings on the lenses will eventually start to craze and delaminate. Glasses don't need to last more than 5 years before being replaced, so it's not as big of a deal. If your telescope became unusable in that time, you'd be furious.

Poly, while being impact resistant, is not nearly as scratch resistant as glass, and is nowhere near as chemically stable. Didn't realize there was dirt on your cleaning cloth? Ruined scope. Cleaned it with regular window cleaner? Ruined scope.

There are other resins that would be better, but nothing nearly as simple and durable as glass comes close to the optical clarity.

Eyeglasses would still be made from glass if they weren't so heavy and potentially dangerous, Not to ignore that nobody wants to wait two months for custom lenses to be made.

Source: I'm an advanced optician running three offices and a lens lab.