Your answer supposes that the OP is not addressing well informed knife people on a knife forum.I am not sure if you are trolling here or not. It is 100% impossible for all the custom knife maker's in the world, to satisfy even 1/2 of a percent of the demand globally for knives. All the kitchen knives, the carpet cutters, the box cutters, the glovebox knives, the old timer folders, the butter knives in drawers etc etc. Most knife makers cannot even handle the small amount of orders they receive comparably, to how many knives their town's Wally World down the street from their house moves on a daily basis.
Why production knives? Because the market, and reality demand them.
Do you really suppose he was asking why Taco Bell doesn't buy custom knives to cut Chalupas?