David made a car, but cars where already around.
It's probably quite obvious where Ridley was going with it. He was going to make the Biomech part of the Alien come from David trying to make an Android type Alien or Mesh his internal workings with the existing creature.
Prolly never get to find out, but that's my guess. If all the material suggests that the Biomech part has been deliberately left out to appear later, then there's not really many explanations. The only other one is possibly that the Engineers have the Original and best Alien, but that would be a mighty coincidence that David magically came up with a beast that looks 98% like their version and does all the same things.