I think the big problem in us being that slow is that you can't properly test changes. The initial programs get written under immense time pressure to put it productive and what after that? Do you want to touch your productive system and refactor it and risk causing hundreds of thousands in damages for production loss? You pretty much need to nail your initial design and can't afford to do the usual "we will take care of our technical debt later".

I hear you saying "but you have debuggers/simulators!", for anyone actually having worked in the field you will know they are pretty much useless for big changes in machines that speak to hundreds of other systems, sensors, motors, etc. At least that's what my experience was and nobody has a backup factory to test changes on. In the tight schedules we had during production stops (Sunday nights, mainly) we were busy enough maintaining everything else than having fun on a PLC.

I completely agree that this is all a big mess and people don't tend to write maintainable programs, they just want their machines running and this is where everyone needs to be trained and improve.