You hire the junior developer because you can get them to learn your codebase and business domain at a discount, and then reap their productivity as they turn senior. You don’t get that with an LLM since it only operates on whatever is in its context.

(If you prefer to hire seniors that’s fine too - my rates are triple that of a junior and you’re paying full price for the time it takes me learning your codebase, and from experience it takes me at least 3 months to reach full productivity.)