Why do mushrooms always grow on wood?

Mushroom is a fleshy, spore-bearing fruiting body of a fungus, growing above ground, on soil, or on its food source such as wood. Mushroom absorbs all of its required nutrients directly from the wood as wood fibers are wet, rotten and mushroom get nutrients without performing digestion of its own. 

Thus it is a saprophyte organism dependent on dead and decay organisms like wood and other plant bodies.