I know that in virtually every case, PCBs start as a substrate with a layer of copper laminated on top, which is then etched away, leaving the traces. But why are the traces not added on an empty substrate instead?

I am well aware that the etching process is standard and very well understood by now, but isn't it a bit wasteful to etch that much copper away (or leave that much unused copper if you're maximizing it)?

Is the reason for this simply that additive manufacturing processes are just too costly, far outweighing the cost of etchant or copper?