Well, in the old times we needed to add DTDs and Schemas (XSDs) to the XML so typing and restrictions were more clear. In XML, all the types are always strings (or elements). That's why JSX has props instead of attributes, so it enhances XML to support other types than strings.

Still, you also may want to add schemas for JSONs, so you can quickly validate its content. But as you pointed out, they are already typed.