OpenAI has 200M users, and solve over 1B tasks per month interactively. This amounts to 1-2 trillion mixed human/AI tokens. The fact is that every user has their own unique life experience, and a reservoir of tacit knowledge they didn't communicate or write down anywhere else. The LLM can elicit that tacit knowledge that would be otherwise lost, it can crawl our minds for ideas and problem solving choices.
LLMs are in a situation of indirect agency. When they propose a solution, the human usually takes it out and implements it in the real world, and comes back for more help communicating the outcomes. Across many sessions it becomes possible to check what AI ideas worked out and what ideas were bad. This is a huge resource, it collects experience from every user. The LLM becomes an experience flywheel, people are attracted to the best models, and they will get the lion share of this experience.
And yes, you can do it with privacy in mind. You can train just a preference model instead of supervised training on chat logs. Just a model that would pick the right answer from a lineup. This way PII and user specifics don't leak.