Commercial Availability of Bare Metal Automation Controllers
I think there are many factors behind it.

Reliability and easy maintenance are very important for the automation industry.  At the same time, factory lines are low volume products, but with lots of capital behind them, so they aren't really cost-sensitive.

But also the harsh reality is that when it comes to technology, the automation industry is far behind other industries. I work with one foot in automation and the other in automotive, so I can easily compare those two - automation is lagging behind by 20 years or so. PLC:s is an old technology and the main reason for using them is probably conservatism.

PLC:s were once much easier to program than microcontrollers. But nowadays we have a standardized C language, awesome free compilers, the ARM market explosion, standardized single-wire debuggers, pre-made code and drivers, the Internet where you can find information and get help quickly etc etc. To make a microcontroller application is much easier today than it was in the 1980s/90s.

And once the development part is over, a custom PCB with a microcontroller costs somewhere around $10-$30, while a PLC costs somewhere around $2000. Eventually even the automation industry will realize this and it will be the death of PLC:s.

As for Arduino specifically, it is of course unsuitable for professional applications, being a hobbyist platform. Using it for industrial applications is nothing but quackery.