Because we pay for the models.

If I pay you for a service, what implicit right should you have to then continue to profit in perpetuity by storing the data I paid you to process?

If LLMs were free your Gmail analogy might hold up. They aren’t, and so it doesn’t.

AI development can continue with the data folks opt into, or with the data AI companies incessantly scrape with reckless disregard for polite system loads. AI development does not require retaining all user inputs forever.