Why aren't glass spoons used for eating? It seems it would solve the problem of metallic taste with some foods.
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
There was never a need, as there was no benefit.
Spoons made from wood, nacre, horn or porcelain were widely available and don't come with the disadvantages of glass, like insufficient resistance to sudden temperature changes or shock resistance.
Borosilicate glass was invented in the late 19th century, so it wasn't available at a time where it might have had a greater influence on culture.
Porcelain spoons are cheap, with no apparent disadvantages, are widely accepted and do match with other kitchenware.