But you do say, “I am building a house.”

Sure, but you don't build a foundation and say "I have built a house."

They're building a game engine, but it is not an actual engine yet. Just one core component.

If what you are building isn't an actual house, or an actual game engine, then how could anybody claim they had build that particular house, or that particular game engine?

If what you were working on--for the entire time you were working on it--was a completely different thing from a house or from a game engine, how could you clame to have built a house or a game engine? Putatively, what you were working on what an entirely different thing from the result.