Question

Why building is called so as it is already built??

Answer 1

it is a human made to build

Answer 2

The word is the result of mispronunciation of [old usage] 'byldinge'.

There could be other reasons too:-

Making it a 'verbal noun', like 'flooring', etc. 

The simple answer would be that the word indicates its state of being. Just like an essay can be called a piece of writing. Although it does sound like the verb building, which indicates an ongoing action, these types of nouns refer to the existence of an object or matter which needs human effort to come into existance and thus achieves a state of being after creation