Not so much that it forgets, its that it reads everything in the beginning. By the time it gets to the part you actually are hoping it gets right, there is too much in the action window for it to see the original prompt clearly; between all the noise with tool calls and tasks being executed the only thing left is a vague recollection of the initial prompt and the last four context windows.

Which is why a hook works where a file wont. It lands when the command is about to run, not 200 turns ahead of time as a suggestion.