I remember hearing somewhere that humans have a limited capacity in terms of number of decisions made in a day, and it seems to fit here: If I'm writing the code myself, I have to make several decisions on every line of code, and that's mentally tiring, so I tend to stop and procrastinate frequently.
If an LLM is handling a lot of the details, then I'm just making higher-level decisions, allowing me to make more progress.
Of course this is totally speculation and theories like this tend to be wrong, but it is at least consistent with how I feel.
While it's often a luxury, I'd much rather work on code I wrote than code somebody else wrote.