Why aren't glass spoons used for eating?
There has actually been some significant research into cutlery being made of the correct substance to match the food

Testing shows perceptible difference in tastes from various metals when combined with certain foods. Just as silver spoons and egg are an unpleasant experience, some metals (e.g. copper, zinc) acted as a catalyst and improved the taste experience of certain foods

We use wood and ceramic for similar reasons, often for the mouth feel of the cutlery and heat dispersion, e.g. hot soups with metal spoons can be painful

Glass would have it's place, but probably not to dissimilar to porcelain (ceramic). Glass spoons are available, but rare



Some references:

Institute of Making - Sensoaesthetic Properties of Materials

Fine Dining Lovers - Cutlery food science