I have some friends in the business of building plants. More often than not, it's build the plant, and you've got X budget, that's it, and have to account for any overruns or delays inside that.
With any left going to the guys who built the plant on contract. They have real good reasons to not waste a dime.
Almost always, there is manual control for things in case some of the automation goes down. The issue even then is there anyone who can run the place manually - once in operation there are usuall too few people trained in the process to run the plant, though there might usually be enough to shut it down safely (depends on how hard that is and is process-dependent).
Of course, once a plant is operational, the bean counters resume control, and good luck with trying to convince them that they need more safeties and backups before the plant has broken even. And once it gets to that point, it's been running awhile without that, so they become even harder to convince to add that stuff - even if it were free, taking down the plant for a day or few to do it isn't free.
Since the line people who actually understand the process are...well, not the most intellectual appearing folks on the planet (they tend more towards being really savvy, but don't impress the suits) they don't have the communication skills to get the bean counters to approve things like this, and we go on, waiting for the inevitable disaster, prevented most of the time only by the savvy operators whose lives tend to be on the line.
This is based on discussions I've had over years with a long term friend and fellow engineer who now builds these plants on contract for big firms, be it oil refinery stuff, ethanol plants, solvent recovery plants or some kind of chemical processing.
He's a good guy, and when he can, applies very high human productivity for the money to make room in the budget for the safeties and backups. But it's not always possible in that game, and I get the feeling he's the exception.
That's one hard business. It can take months to do all the work to bid on a fixed price contract, and he's been sued for backing out of one when the price of the stainless steel doubled during the time after he bid and before the bid was accepted.
But he had no choice but to walk away -- he couldn't come up with the difference, and saw it as more honest to just not accept the job after all.
The court agreed -- after considerable time and legal expense. Fortunately most customers are a little more reasonable, but not all that reasonable, if you understand my meaning.
I had pointed out a couple years back (maybe 3-4) how vulnerable SCADA systems were, and Bruce poo poohed me then, saying nothing was Internet connected. Well, I was right then -- and it was connected then, so the bean counters could watch the plants run in real time, change thing (sometimes causing issues), and require less on site paid workers to get the job done at the highest profit.
At least in the earlier days, most of this was done over dedicated lines, but the switch to the net caught out Bruce just like grocery store scanners surprised Bush 1....