Wood is a forgiving, strong, and durable material for supporting a lot of weight, mechanisms, screws, staples, etc.
Plastic is not.
Interflip dragon is actually a hybrid playfield. The bottom 3/4 is wood, and the surface is a layer of plastic with the artwork reverse printed.
There were other experiments done by other major manufactuers with plastic playfields, but they never went into full production.
However, it seems like the future of playfields may be the hardtop overlay concept. I wouldn't be surprised if manufacturers started considering this.