Why are refrigerator shelves mostly made from plastic or glass?

Only modern fridge shelves are made of glass and plastic - because its cheaper. Traditionally fridge shelves were made like metal wire grills. This allowed the cool air to circulate from top to bottom. If your fridge has impervious plastic or glass shelves you must be careful to leave space at the front a back for air-flow. Very rarely do we see something new that is better. Mostly its just cheaper and poorer quality but the sales pitch is that its new and better. Yeah right!