I believe, the primary catalyst for Meta to build Threads is competing with Google and Microsoft on LLMs. Google Groups and GMail have nice and clean conversational data, while Microsoft has that via LinkedIn (and to an extent, GitHub). Short of Facebook and Messenger, Meta has to license from Twitter and Reddit, but might as well try their luck with Threads instead.
I believe they will eventually change WhatsApp's privacy policy to mine the data in there, as well, with the help of "differential privacy" or something, like Apple. Mark is too smart to not to.