Because cheap and quality code is only part of the story. The code needs to solve the right problem and that is a domain only a human can operate, at least for now. Back then when I was inexperienced I couldn't write good code, but I could sit with the company's CTO while he explained the domain, the challenges and the goal of the project. I could talk with domain experts and understand what the common solutions to the problems were. These are things that for an LLM to do would require untold amounts of context or a specialized model that understands the domain.

But the thing is, there are many unknowns. We humans are very capable of adapting as we go. LLMs have a fixed data they were trained on and prompt engineering can only get you so far.

I think anyone asking this with the intention of actually replacing humans with LLMs don't really understand neither humans nor LLMs. They are just talking money.