Oh course it's not black and white, there are many shades of grey in how detailed the design of a feature can be. Often even if I know low level details, I'll only give the AI high level requirements because I want to see how it would do it. Often it comes up alternative/better ways of doing what I planned and I incorporate those ideas into the final design.

The designing is the hard part. Writing code from a comprehensive design spec is a small part of the task.

So, people do know how to design a feature, but they also know it takes a lot of time and effort. They want AI to do that work for them.