I think OP's issue is that celebrities are far more likely to get books taken on by publishing houses, given huge promo budgets and used as The Face of a publisher, when their books may be sub-standard or just not as good as other books that don't get given the same advantage or push.
The books sell because of who the author is, not what the book is like. I have no axe to grind with DW because I don't write children's books, but the same issue is rife in adult publishing. A TV 'star' is 'told' by their agent to write a book, gets a ghost writer in who is paid peanuts, turns in a standard 'written by numbers' book and the publishers pay to get that book to the front of best seller lists and bookshops.
It's not a level playing field.
But it is what it is and lots of us write because we love it, our publishers like our books but don't have the budget to put us all on the side of buses, so we sell in a trickle and make peanuts. And in the meantime, the rich get richer.