Why are most industrial chemical processes thermal, instead of photochemical?

Answer 1

Thermal energy is much easier to control in industry. It is also easier to produce. Photochemical would require intense lights of the correct wavelength which would also require more electricity.

Answer 2

I doubt that the majority of industrial reactions are of the kind that can be initiated or promoted by visible or UV light. Examples - production of superphosphate, gasoline, sulphuric acid, alumina, steel, other metals, sodium hydroxide, sodium cyanide and dozens of others.

Some reaction in the pharmaceutical industry involving steroids are or can be promoted by UV. Some plastics as well.