I think it's that 1) the cost of entry is minimal 2) should it become a hit, the rewards are great and 3) already being a person of renown means you'll be allow

I think it's that 1) the cost of entry is minimal 2) should it become a hit, the rewards are great and 3) already being a person of renown means you'll be allowed to try. Your competitors (unknown illustrators and writers) are kept out by the book industry unless their work speaks for itself. There are a lot of children in the world, and they like books, and their parents buy a lot of them. Children like both novelty (so new market entrants always have a chance) and familiarity (so if your book is a hit, it'll sell continuously for years and there'll be demands for sequels, TV series, live action film, toys, etc) Have you ever wondered why celebrities also launch perfumes, clothing lines, pasta sauces, actresses launch a singing career and vice-versa? The celebrity has run a gauntlet of vying for attention and has already won; having some iota of Star Power, they can command some attention and persuade the public to buy some things. That is lucrative and thats what marketers are looking for. It reminds me of an old article that was a step-by-step guide to a career as a voiceover artist. Step 1 was "become a celebrity with a distinctive voice (admittedly this is the hard part)", step 2 was "accept the offers and turn up at the recording studio, make sure there's plenty of room in your wallet"