Seems like there is an opportunity here? Presumably milling machines and so on will start to reduce this overhead, so that the overhead becomes just the cost of creating the original design?
Come to think of it, there might be some opportunity in standardization. I'm not an aviation engineer so what I say may make no sense, but a good enough set of "bricks" that would let you assemble anything from small twin-seater to a passenger plane would benefit from economies of scale, even if initially such a design would be more expensive and less performant than a custom build.
I guess since it isn't already happening, I'm probably missing something obvious.
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.
Helicopters are extremely weight limited. In order to allow for any sort of useful payload, all of the airframe parts have to be designed for a specific model (or range of very similar models). The shapes and mechanical properties needed aren't well suited to highly automated mass production.
Avionics and engines are somewhat standardized "bricks", though.
I guess since it isn't already happening, I'm probably missing something obvious.