They're not. Both the UK and the US have long and strong traditions of publishing great books for children of all ages. And so does much else of the world, but British and American children don't get a chance to read them
Why are all the good children's books British? When I browse children's books here in the States it feels like a disproportionate number of them are by British authors (I don't notice this with adult fiction, but then I read less of that).
Well, the short answer is that they are not! If it feels like that now, that is just a fashion. British children's books are excellent but, so too are many published in the US. When I was a growing up and all through the 1970s and 1980s the US produced teen fiction which was the envy of the world, with authors such as Robert Cormier and Paul Zindel writing effortlessly and with deadly accuracy about the complexities of home and school. The US authors were even more distinctly way ahead in the slew of humorous novels about serious subjects including Betsy Byers's The Eighteenth Emergency and Florence Parry Heide's The Shrinking of Treehorn. And the US publishers and children's writers were also busy moving forwards; it was in the US that the major issues of racism in children's books were first tackled with novels such as Paula Fox's The Slave Dancer, Mildred D Taylor's Roll Of Thunder, Hear My Cry, Rosa Guy's The Friends and many, many more. In picture books too, the US produced classics which haven't readily crossed the Atlantic such as Margaret Wise Brown's Goodnight Moon as well as many which have and have lasted equally well here such as Maurice Sendak's Where the Wild Things Are. And then there is The Very Hungry Caterpillar and so on. The lists of US books showing success in both markets is endless and they come right up to date with books such as Jeff Kinney Diary of a Wimpy Kid, Dave Pilkey's Captain Underpants or Rick Riordan's Percy Jackson riding high. Both the UK and the US have long and strong traditions of publishing great books for children of all ages. And so does much else of the world but few British or American children get a chance to read anything that isn't originated in English. Why? Now there's a question