Question

Why is a building called a building if its already built?

Answer 1

A building would be called a building mainly because this is what it is called. It's simply called this. You can't really change what and how something was changed. But, it's true, a building is already built, and it's still called a building. But, I believe that it would be simply called this.

Answer 2

this is just a guess. but perhaps because the building never really stops. any structure tends to decay/breakdown/fall into disrepair over time, thus it is in need of constant maintenance (or additional "building"