There is an option in ChatGPT that you can use to turn that off.
> Chat History & Training Save new chats to your history and allow them to be used to improve ChatGPT via model training. Unsaved chats will be deleted from our systems within 30 days.
The default is your chats are their property to train on and use as they wish.
If you dig into the Settings you can disable sharing chats with OpenAI, but you lose access to just about every feature including saving chats. You can only have one chat at a time, and if the window is closed or refreshed your chat is wiped. it's kind of like if opening a "Private Browsing" window prevented you from having a regular browsing window open and also had no tabs.
For some reason, they still retain your chat for "up to 30 days" despite not letting you save or access it after the page is refreshed.
The 30 days part is most likely a legal compliance bit to cover their asses if their backing data systems ever take a big dump.
They need to use an asynchronous system to be tracking when your chat becomes "finished" and then likely queue it up to a system looking to propagate deletes. They have to choose some kind of SLA on that and probably went with a common data privacy user data deletion window of 30 days.
> Chat History & Training Save new chats to your history and allow them to be used to improve ChatGPT via model training. Unsaved chats will be deleted from our systems within 30 days.