What temperature is really required to make fused quartz?
Glass is not necessarily made of pure silica $\ce{SiO2}.$ Ordinary glass is a mixture of at least $50\,\%$ $\ce{SiO2},$ plus smaller proportions of $\ce{CaO},$ $\ce{Na2O},$ $\ce{K2O},$ and $\ce{Al2O3},$ with impurities like $\ce{Fe2O3}.$ Pyrex has also some $\ce{B2O3}.$
Glass has no defined melting point. It becomes like a very viscous paste at around 800 °C. Mixtures have melting points that are always much lower than their components.