Part of the PCB manufacturing process involves bonding copper foil onto the substrate. This is hard enough to do well when it's done across the entire panel.
But why are the traces not added on an empty substrate instead?
Because the traces are tiny, and need to be bonded onto the panel in such a way that they are laid out accurately with respect to one another and won't come off with soldering or mechanical stress. Short answer -- no one has figured out how to do it economically.
There are PCB manufacturing processes that involve printing a PCB with conductive ink. I don't think I've ever seen such a board, even in a cheap consumer-grade device. Simply based on the fact that it hasn't taken over the world, I think we're back to the notion that etching is easier.
isn't it a bit wasteful to etch that much copper away
Yes, it's just a little bit wasteful. But it's a lot less wasteful in commercial operations, because the copper can be reclaimed from etchant in a way that also regenerates the etchant. For the price of some electricity, the PCB manufacturer has some copper bars to sell back up the chain, and doesn't have to buy as much etchant.
Or leave that much unused copper if you're maximizing it?
Usually when you leave copper on a board it has nothing to do with concerns about using up copper or etchant -- it has to do with using that copper as a ground plane, or bus bars, or some other reason that you want to have copper on the finished board.