Why is it called a building if it’s already built

Because the 'ing' in the English language is added to certain verbs to make them nouns.

Explanation:

Hope this helps :D

Explanation:

It is called a building because the ‘ing’ in the English language is added to certain verbs to make them nouns. The verb ‘built’ is suffixed with ‘ing’ to make the thing a noun- Building.

Reason: The act is of building it when the actual work is in progress. Building is a gerund- a noun made from a verb. Any action verb can be made a gerund