Because treating Plastic like a smeltable, real-life Ore is pretty hilarious. It doesn't really work for wood. "Hey, I just hacked this tree trunk out of the ground, it's a Wood Ingot!" ...it's just not the same.
The simplest answer is that there is no source of wood underground, wheras ores and even plastic (made of fossil fuel) can be found underground. You could argue that furniture and doors are made of wood, but... *shrug* In any case, a joke played too many times is a joke that's lost its charm. Plastic ingot is amusing. Stretching the joke to include wood ingots, stone ingots, well... it ceases being amusing.