Because there's nothing to "move up into" other than "spec, optionally design".
Because the jobs that had "spec, design but don't code" already existed for decades, and pay less than half what the "design + code" person earned.
"we have service A that receives a request, it now has a new flag in it, we need you to pass it through to in the call A makes to service B, and then add it in the where clause of the query that B makes".
and expect it to take 2 days including manual testing.
Now you would expect the same much quicker. Any weird bug of the kind "flag not showing up in B because its another weird place where the request _actually_ goes through" that would before suck up 5 hours, would now be found out by the LLM in 2 minutes. "Oh because of this feature being activated in <random yaml file>, this new path is used, so you have to add the flag passing logic there". And the next day they get a new task.
This was an extreme example, and it's also not a silver bullet, since now you need to ensure that the intern does the task in a way that they still learn the codebase and the service structure (ideally, they learn quicker) and doesn't become completely beholden to the LLM. So that will also become a skill teams look to hone, how they use tools like this.