Instead of real glass, which is fragile, they could use reusable acrylic glasses. They could still stack together to save space. I would rather eat off sturdy plastic dishes than flimsy ones. I suspect the reason they don't do this already is because they don't want to have to deal with washing everything.

Also, real glass is heavier. Every pound they carry is amplified by the need for more fuel on the flight.

I can't recall the last time I flew and got more than a cookie or salty snack. I guess overseas flights may still get plates.