Because it's not really something that matters for the vast majority of consumer photography equipment.

https://istarscopeclub.proboards.com/thread/247/twisting-fac...

^Some 2012 forum post claiming the material cost $2000/kg, although their wording is ambiguous on whether this was the price for the raw material or the finished lens. But based on the manufacturing process described in the source link of this post, it would appear to be a much more cumbersome and complex process than manufacturing regular glass lenses. Fluorite lens are also more fragile which may impact yield %.