I know machinists and welders. Some of them just want a blueprint and will make what you want to that spec. Some of them want to be told "make this part fit on this other part", and they get to make up the bracket that fits. There is a place for both. One is basic and easy to automate - but often it turns out the costs of setting up automation is higher than the cost of paying someone to do the production run. The other is hands on engineering and needs some engineering knowledge.

Can you put programmers in a production line and expect quasi linear improvements?

No.

Because programmer jobs have a knowledge and creativity bottleneck, that is the hard part to scale, and require workers that have autonomy, can take initiative, and can put together a solution from known part in ways that create solutions for one off problems.

Metaphorically speaking they are not tightening the same bolt type over and over again to the same spec every time. Software is not manufactured, it's largely artisanal.

I think those defects often come about because the rest of the process emphasises schedule over quality. It’s a false economy, but many businesses treat quality as a cost rather than a benefit.

It is again one of the problems with scrum. There is pressure to complete a feature in a given sprint - and a sense of failure if you don’t.