For the same reason we do not say "a pure money" - the noun is uncountable.
"Glass" can refer to a number of things. The most likely reference in your quote is the material glass (def. 1 at OALD), in which case it is uncountable. In other cases, e.g. "a glass of milk," it refers to the container for the beverage, in which case it is countable.
But there are exceptions. We do come across uncountable nouns that are sometimes affixed with indefinite articles:
Pine is a soft wood.
"Wood" as used here (the material used in woodworking) is uncountable, but the indefinite article is used because we are discussing a type of wood.