Why the word building is used insead of using built?

Solution 1:

It is a verbal noun. Also, building does not necessarily connote a finished structure, it can mean an unfinished structure too.

The origin is from Middle English around 1250–1300, from the word byldinge.

Source: http://dictionary.reference.com/browse/building

Solution 2:

There are examples of what you're looking for - the built environment; the written word. In these cases, the completed form of the verb is used as an adjective.

In the declaration, 'It is written!' Or 'Let it be written!' there is also a clear sense of completion intended.

In the example you gave, 'building' is a noun, commonly referred to as a 'gerund', or 'verbal', a word derived from a verb, often used as a noun. The forms refer to 'the act of doing something' rather than the process of doing it as an activity through time. In this sense, a building (structure) is the product of an act of building (activity). As such, it stands for the act which produced it, and becomes a rather beautiful metaphorical use of linguistic thought.