The situation is not truly comparable, tbh.

Artists very much retain legal rights to the art they create. Hence the current lawsuits against various AI companies. Meanwhile it depends on jurisdiction whether a comment/thought you write on a public-facing website can be considered your legal production for a civil lawsuit. It’d be trivial if it were a closed site with a very selective admission process with some easily evaluated barrier (say, only people who study at university XYZ are allowing on the otherwise private forum of that university), but public-facing it’s more ambiguous.

You can still try to sue someone who taking that content, but it’s not as clearcut that someone violates your rights as with artists and their art. Meaning that there’s less basis for someone wanting this to always have to be explicitly opt-in and get explicit permission. At least right now. This might very well all change as a result of AI lawsuits.