The reason for things you've described is that LLMs are forgetful. They just can't remember the context and have to research the code almost every time you prompt. Even the code it itself wrote. This leads to re-implementation of the same features with different code, code duplicates, missing the implementation of corner cases, etc.

Yes, this is very close to what I meant. They keep reconstructing the codebase from fragments instead of actually carrying the system forward, so you get re-implementation, duplicates, missing edge cases, and slow architectural drift.