[bluGill]

Two obvious things:

Weight and balance is critical for airplanes. You can design the bricks, but you can't stick them together in any combination because a brick that will hold the tail wings for a 400 passenger plane is too heavy for a 100 passenger plane: the plane will not balance right and always be in nose up stall.

Also, there are not enough airplanes made. Even if the bricks idea can work, we still wouldn't make enough to give it significant enough economics of scale.