There might be a good reason why they use glass ceramics instead of for example stone. Maybe because it's harder to manufacture consistently (ferromagnetic impurities?), has a larger coefficient of expansion or because it's harder to implement touch controls for them. It might just be more expensive.

Ceramic glass is easier to make, diamagnetic (which means more magnetic flux kept towards the tubes for better efficiency,) and also easier to make white-colored so you have better reflection. Cheaper than using aluminum.