From what I know, he never really wrote “for” the silmsrillion either. He wrote stories for him to flesh out the history of the world but not with the intention of publishing such stories. Some of them were even just notes about what happened in the world and some weren’t finished.
Someone correct me if I’m wrong
According to the Tolkien Professor (during his YouTube streams on the History of Middle Earth series) there was always the intent to publish the Quenta Silmarillion (the central tale of the Silmarils) as a First-Age story of the Elves, but it kept getting revised and rewritten and never reached a publishable form.
Until Tolkien’s son wanted to complete that piece of the legacy, and found multiple (sometimes contradictory) sets of notes and mostly-finished stories, and Editorial Decisions had to be made.
