Question

Why is a building called building when it is already built?

Answer 1

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. 
It could also be a suffix for present participles like 'singing', etc.
It could be a noun from a verb too.

There are other reasons (not necessary) which I didn't mention, but if you need I will  ^ ^

Answer 2

The answer is simple...  it is a noun... the gerund form of verb being used as a noun.  The result of acts of building are called a building.

If it meant to be a funny or tricky question, then I suppose there is no easy answer to that.   Perhaps it still builds something...