That's because many developers are used to working like this.
With AI, the correct approach is to think more like a software architect.
Learning to plan things out in your head upfront without to figure things out while coding requires a mindset shift, but is important to work effectively with the new tools.
To some this comes naturally, for others it is very hard.
I think what GP is referring too are technical semantics and accidental complexity. You can’t plan for those.
The same kind of planning you’re describing can and do happen sans LLM, usually on the sofa, or in front of a whiteboard. Or by reading some research materials. No good programmer rushes to coding without a clear objective.
But the map is not the territory. A lot of questions surface during coding. LLMs will guess and the result may be correct according to the plan, but technically poor, unreliable, or downright insecure.