I have a pet theory. LLMs are trained on many successful open source projects. Due to their success, the projects are used very widely and they cover extreme edge cases which have accumulated over many years and which are usually not covered, and don't need to be covered, in majority of regular software. So, when LLMs write code, they try to mimic the code they have been trained on and incorporate these edge cases into each fragment they write. It results in more code which is also harder to read and comprehend.