It just isn't feasible to produce the temperatures on Earth which can be easily done with giant solar ovens and containerless processing in zero gravity. Even medium temperatures are easier to produce in space. Giant solar ovens can be built in zero gravity and with no wind. These can be relatively lightweight structures, e.g., foil mirrors. Zero-gravity containerless processing using very high temperatures will usher in a whole new field of materials science processing and manufacturing impossible on Earth.
Thermal energy is cheap and clean in space.
The oven used lunar simulants to produce cast basalt rods, bricks and glass fibers. Standard ASTM tests found that the rods had compressive strengths of approximately 10,000 p.s.i., which is about two or three times greater than concrete. Another welded two bricks together by putting lunar simulant between them and heating it, producing a weld depth of range 1.3 to 1.9 cm. Further studies were underway on crystallized cast basalt structures and glass composites. Additional work was planned several years ago.