Whereas to a Brit that sounds like saying “plates are normally square”!
I guess because casserole (the food) is named after the vessel, and pots & pans are traditionally round, the whole “big pot of stewed stuff in the oven” meaning of casserole is still central to British usage, so the image of the dish is round (or oval - but still rounded). Dutch Oven is not a common term in the UK. I mean, it’s known, but it’s not standard. I only really know it from American cookbooks and food shows, and I’ve been cooking since I was a kid.