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Didn't he write the Hobbit first and then everything else around it?
No, not really. It was his first book that was supremely popular, but it was written for his children. His main body of work (which was later published in part in the Silmarallion) was started in WWI and was never really completed. The Hobbit and then to a far greater extant LotR were pulled into the preceding work.
He already hada lot of stories and ideas about Middle Earth before. When he wrote the Hobbit for his kids, he placed it in this world and it became the first book to be published. Lord of the Rings he wrote as a sequel to the Hobbit, but added a lot of hints and references to his other stories of his world.
He set The Hobbit (which he wrote for his kids) in the world he'd already built... not because he particularly enjoyed worldbuilding, but because a culturally complex fantasy world with a rich history and mythology was a prerequisite for the epic poetic sagas he felt needed to write in order to properly develop his fantasy languages, which is what he really liked to do, as a philologist.