Because the word 'building' can be either a noun (naming word) or a verb (doing word).
When the term 'building' is used in the context, 'she is building it', then the word is a verb. An action is happening.
When it is used in the context, 'look at that building', the word is a (common) noun. It's the name of something.
This means that the meaning of the word 'building' is context dependent; therefore, there isn't a relation between the noun and verb form. Hence, the fact a building has already been built has absolutely nothing to do with its name.