I would guess that this would be much more expensive than just CNC machining.
Especially with the size and geometry that some of the higher end boards have on the inside.

And it would probably still required at least some finishing in a CNC machine.
Screw holes will have to get drilled and tapped and the mating surfaces between parts have to be absolutely flat to not create any visible gaps in the assembled part.

And with casting the QC process is usually much more involved than with CNC machining since a casting can fail a lot easier than a CNC program.