Because it's good luck.
People knocking on wood is a superstitious act. People think that they say something bad and then knock on wood, it will not happen to them.
Touch or knock on wood is an old superstition based on the belief that spirits resided in trees and plants. You touch wood (some people say knock on wood) to ask for good luck or the spirit's blessings.
The phrase 'knock on wood' is believed to be Germanic in origin. In history it was believed that tiny but lucky creatures lived in the wood. Knocking on the wood would bring them out to gran you good luck.
Knocking on wood is an old superstition. Originally you were supposed to knock on oak wood. This was to awaken the spirit of the wood and ask it to protect you from whatever bad luck you had inadvertently aroused. Three is a lucky and in some places sacred number and I was raised to believe you should knock three times to avert bad luck.
you knock on wood because say you say something like iv never broken a bone in my life you would say knock on wood and knock on wood 3 times so you dont jinx your self and break a bone tomarow. The origin of "knock on wood" is a bit cloudy. It seems to trace back to the European idea of touching wood to ward off bad luck, or acknowledge good luck. The knocking probably stems from the idea of a knocking sound preventing the Devil from hearing unwise statements. The modern American usage began early in the 20th century. There isn't any specific thing to do (you don't have to knock specifically three times as cited above), but the act is common.
instead of knock on wood, it can be knock on snow!
Probably from sailors who would "touch the (wooden) ship" for good luck. Without a ship to touch, landlocked sailors may have used a piece of wood as a symbolic one.
It was believed that trees had spirits and to invoke those spirits you would tap on the tree, "knock on wood". The practice tended to be a way to ask the spirits to intervene and save you from a bad out come (bad luck). This is why when you say something you do not want to come about you "knock on wood."
Knock on wood means you are hoping that it happens.
The exact origin of the phrase "knock on wood" is unknown, however many speculate that is could have come from rituals in early times where trees were considered sacred spirits. Others believe the phrase originated in Ireland. To knock on wood was to alert leprechauns that you were thanking them for giving you good luck. Another popular belief is that by knocking on wood you prevent the devil from hearing what you have just said, which in turn prevents the action from occurring. It is also possible that the origination of the phrase "knock on wood" is derived from Christianity and the wood is a reference to the cross on which Jesus was crucified.The British version of this expression, 'touch wood,' is supposed to have derived from a 17th century practice of warding off bad luck by touching something made of wood while saying 'absit, omen!'Phrase maven Gary Martin (please see link) says it became 'knock on wood' in the United States by the early 20th century.
Knock on wood means you are hoping that it happens.