Flexible glass was invented around 20 AD. So why have we never seen it?

Flexible glass is a legendary lost invention from the time of the reign of the Roman Emperor Tiberius Caesar (14–37 AD).
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The craftsman who invented the technique brought a drinking bowl made of flexible glass before Caesar who threw it to the floor, whereupon the material dented, rather than shattering.
The inventor then repaired the bowl easily with a small hammer. After the inventor swore to the Emperor that he alone knew the technique of manufacture, Tiberius had the man beheaded, fearing such material could undermine the value of gold and silver.