Siri wasn't originally intended to be a chatbot. That's a goal that was grafted onto it only upon public release of ChatGPT in late 2022. Siri's original main functionality, which for about 11 years wasn't supposed to be dependent on an LLM, was to have knowledge of your personal data across multiple apps, and the ability to pull in other data, to allow you to tell it to set schedules and appointments, pluck relevant data out of your emails, coordinate flight arrivals to allow you to meet someone at an airport, plan your trip to the airport and other trips, control some Apple products and appliances, etc. It can do many of those things, but so inconsistently for many people that they can't rely on it. ChatGPT can't currently do many of those things since it doesn't have access to your personal on-device info.