You’d be surprised how much industrial automation is not built by relatively small contractors. To your point, they tend to use “mega-corp” systems (e.g., Rockwell Automation hardware) but the actual logic is often written by small companies that bid out the project (or are subcontracted to for various reasons not necessarily related to technical acumen).

I haven’t personally worked o power plants but have been involved in various safety critical automation builds related to factories, hospitals, high energetic test stands etc. often, the code is maintained by a technician who isn’t either an engineer or classically trained computer scientist. Not to say those credentials are a must, but it makes more sense why the code I witnessed had so many bad practices in it