Why do you still call it a building, when it's already built?

It's a standard formation - compare a painting, a carving, a drawing, also a reading, a showing, etc.

Because the Middle English word "byldinge" from which it derives was not a gerund. A very commom misconception - and very popular as a riddle.

Some people think that a project under construction should be called a 'build' - but then some people don't know what they are about.

ha ha