It turns out there's a massive speed advantage to parallel processing that outweighs the cost of making the molds when you're making hundreds of the same item. Look at a machine that makes plastic bottles - it stamps out something like 1 to 10 bottles per second per machine, by inflating a bubble of plastic inside a mold. You will never ever get that speed with something that only prints one part of the bottle at a time.

Resin printers are kind of parallel but I don't know how much faster they are. And they still won't beat the bottle molding machine.

For serial processing for expensive parts made of strong materials, wasn't there already CNC milling?