Because AI doesn't "understand" natural languages. Writing software is about converting some understanding of the world ("I want to be able to solve this human problem") into unambiguous specifications (usually that's a programming language).

Sure, it can auto-complete such that the developer does not have to remember anything (which is arguably making developers worse at doing their job), but it cannot understand the reality of the person who will be using the program.

It is already a problem with human developers who write programs that they don't use themselves, solving problems that they don't understand themselves.