Because that requires breaking the almost assured stipulation in a software license agreement that says "You will not try to RE this software", which is fine to ignore for home gamers, but not something you generally want to mess around with in a business setting where the risks are much higher.

The reason industrial PLCs cost so much is because of their reliability. They need to be working flawlessly 50 years in the future. Trying to vibe code your way into a PLC compiler feels like it could blow up in your face after a little while.

That is also ignoring the safety PLCs with safety instructions that are certified for use with ISO 13849. Can you imagine getting inspected because your safety subsystem cause injury and you explain how you don't use the officially licensed software to program it.

For personal project, that could be fun however.

That is only recently become an option you could try, and we don't know long term how well it will work. Remember laws may change (and there are some legal issues around LLMs that are not clear at all)