I'm pretty sure there's either a Paul Graham or Joel Spolsky essay on the subject, but basically: What's the difference between a sufficiently complete and formalised specification, and a high-level programming language?

Domain specific knowledge?

An AI would know how to fill in the gaps when you said "Make me a blog page" or "Make this server scalable."

This may sound outrageous, but some very large percentage of coding practice is wheel reinvention. Developers the world over solve the same problems over and over again.

One of the first applications for useful AI will be to collect and automate that repetition.

There's already a "make me a blog" project, and Bootstrap is so cliched now I'm surprised there aren't more site builders being developed. So this isn't entirely impractical.

The hard part is the visual design. It's much harder to automate that in a way that includes some genuine original creativity.

Of course web apps != general scientific or business coding. So AI isn't a complete solution. But it doesn't need to be to be useful. (Domain specific, after all.)

Download the Tokeneer project's Z specifications and Ada source code to see the difference. I'm sure it will nicely illustrate it.