I'm really buffled what makes it for Apple so difficult to cut off Siri and replace it with state-of-art technology (LLMs). Siri is still akin to a "dumbphone" 4 years into the age of "smartphones" which is a good indicator for a lost case and when it's better to start a product from scratch without the tech debt.
Org level dysfunction. It is likely an exec's pet project.
Kind of like how Meta keeps the disaster that is Horizon Worlds going, but much worse because Apple has a monopoly on AI integrations in iOS, and nobody else is allowed to compete with their terrible implementation.