I too have wondered why it take so long to make stuff in CAD, and why even suggesting that everything should fit together the first time is laughable at best.
My theory is that computers can't do rough sketching. No CAD software suite (I think) can iterate and evalute rough ideas as fast and flexible as whiteboard pen in a meeting room can.
Just as an example I am familiar with: so many people appear to begin a project like a MAME cabinet with SketchUp.
I like "Cardboard Assisted Design" and have literally built several MAME cabinet prototypes in cardboard where iteration is easy with merely a box-cutter knife.
When the ergonomics and part-fitting is "go", I take measurements from the cardboard proto and move to wood.
Designing acrylic parts for later laser-cutting I have also used "CAD" for prototyping — sometimes even flat-bed scanning the chipboard prototype and then moving to a vector drawing app to overlay the laser-friendly beziers.
Even for PCB layout I often will laser-print the PCB as a final sanity check: punching holes in the paper and shoving the actual electronic components in to see that everything will fit before I send the file off to Taiwan.
Your theory seems sound. I see this theme of free-flow expressivity vs stricture and formalism in the choice of programming languages. CAD is like rust, where it never fits together the first time and parametric stricture lines the road of progress with infinite caltrops in abdication to correctness; javascript is a sketch book where ideas and mutations happen easily and often, correctness is a beautiful illusion brought through the evolutionary nature of experimentation.
Mechanical CADs aren't designed for maintainability.
They are designed for expressivity first, and easiness of learning (for people with industry knowledge) second. And those are the two only goals.
Just try to adapt a mechanical design for a slightly different task. It's usually better to start from scratch.
(Anyway, that's an example of computers not being fully used. Going from the Solow paradox into "computers are bad" - like lots of people like to do, even here - is just stupid.)
My theory is that computers can't do rough sketching. No CAD software suite (I think) can iterate and evalute rough ideas as fast and flexible as whiteboard pen in a meeting room can.