Because from my experience using codex in a decently complex c++ environment at work, it works REALLY well when it has things to copy. Refactorings, documentation, code review etc. all work great. But those things only help actual humans and they also take time. I estimate that in a good case I save ~50% of time, in a bad case it's negative and costs time.

But what I generally found, it's not that great at writing new code. Obviously an LLM can't think and you notice that quite quickly, it doesn't create abstractions, use abstractions or try to find general solution to problems.

People who get replaced by Codex are those who do repetitive tasks in a well understood field. For example, making basic websites, very simple crud applications etc..

I think it's also not layoffs but rather companies will hire less freelancers or people to manage small IT projects.