Where do you go when you need just a substrate? You don't care about its chemical composition, or crystallographic structure, electrical conductivity, etc. You just need a thin, very clean, temperature resistant, chemically resistant, mechanically sturdy piece of material featuring very smooth surface. And you need, let's say, 150 cm2 of it, or may be even 500 cm2, and you don’t want to spend thousands of $$ to get it. And, in addition, you want it to be compatible with a standard photolithographic pattern definition tools.
The answer is silicon. Yes, only silicon wafers meet all those requirements. No glass, no quartz, sapphire, any ceramics, metal, other semiconductor wafers…. Yes, only silicon. And you can make into an insulating substrate by easily growing thermal oxide.
I realized all of the above when looking for the low surface roughness, temperature resistant, thin piece of a solid to run some thin-film deposition studies in my group. What it is chemically did not matter. To my surprise silicon turned out to be the only solution. Long live silicon!
