Why do we call it a BUILDING? (when it is already been BUILT)

Because Building as building is a Noun (a word that can be used to refer to a person or place or thing ) while building as constructing is a verb ( a content word that denotes an action or a state).

I didnt give the meanings of the words to suggest your unintelligent or anything, i just wanted to clarify it all, to give my answer a bit more depth:)

The word describes this definition very well:-

the business, art, or process of making houses, stores, bridges, ships, and similar things.
Ex. The boom in building ... has resulted in the local land being more valuable now for houses (London Times).
(Abbr:) bldg.

Because a building is never completely built.

Many years ago, a fisherman's wife blessed her husband with twin sons. They loved the children very much, but couldn't think of what to name their children. Finally, after several days, the fisherman said, "Let's not decide on names right now. If we wait a little while, the names will simply occur to us."

After several weeks had passed, the fisherman and his wife noticed a peculiar fact. When left alone, one of the boys would also turn towards the sea, while the other boy would face inland. It didn't matter which way the parents positioned the children, the same child always faced the same direction. "Let's call the boys Towards and Away," suggested the fisherman. His wife agreed, and from that point on, the boys were simply known as Towards and Away.

The years passed and the lads grew tall and strong. The day came when the aging fisherman said to his sons, "Boys, it is time that learned how to make a living from the sea." They provisioned their ship, said their goodbyes, and set sail for a three month voyage.

The three months passed quickly for the fisherman's wife, yet the ship had not returned. Another three months passed, and still no ship. Three whole years passed before the greiving woman saw a lone man walking towards her house. She recognized him as her husband. "My goodness! What has happened to my darling boys?" she cried.

The ragged fisherman began to tell his story: "We were just barely one whole day out to see when Towards hooked into a great fish. Towards fought long and hard, but the fish was more than his equal. For a whole week they wrestled upon the waves without either of them letting up. Yet eventually the great fish started to win the battle, and Towards was pulled over the side of our ship. He was swallowed whole, and we never saw either of them again."

"Oh dear, that must have been terrible! What a huge fish that must of been!"

"Yes, it was, but you should have seen the one that got Away...."

I think because it's a big and elegant thing