This is an optics issue. The development of the techniques to create glass of sufficient purity and clarity with very specific attributes and then shape it into a lens is an incredibly sophisticated, high value technology. The Venetians (the best glassmakers in the world for much of this period) were so secretive about this technology that they sequestered glassmakers on and prohibited foreigners from visiting the island of Murano where high end glass was made.

Even today the making of lenses is still considered a secretive and controlled technologies with one single firm, Essilor, controlling almost 40% of all glasses lens production in the world.