I think it has to do with mental model. If you already know what to write and it is reasonably complex you'll have a mental model ready and can quickly write it down (now even faster as LLMs autocomplete 3-4 lines at a time). While reading someone else code you'll have to constantly map the code in your mind with code written and have to then compare quality, security and other issues.
I just tend to find LLM code output extremely to read, I guess. It tends to be verbose and do a lot of unnecessary stuff, but I can always get the point easily and edit accordingly.