I'm only involved in manufacturing as a hobby for my custom keyboard projects. But my bet would be that it's about injection molding [0].
Once you got the molds machined you can mass produce plastic parts extremely cheaply. Need more plastic parts? Get a few more molds made. It scales pretty well because one mold can produce multiple parts at once and the injection molding presses itself are relatively inexpensive and pretty much fully automated.
Wood while being great for prototyping doesn't scale that well in mass production. You would machine it using CNC routers and if you need more produced you need new expensive CNC routers which can only produce one part at a time. Also the CNC routers I know you have to baby sit.
Again: That's only from my limited experience with limited runs (a few thousand pieces at most). Maybe someone who knows more about real mass production can elaborate a little more on this.