Because UI toolkit independent APIs are more flexible than just returning HTML, and considering only HTML means that you offer subpar experiences on most platforms. Not just mobile software, where web apps are awful, but also desktop, where your software doesn't integrate well with the platform if it's just a webpage.

Returning purely data means being able to transform it in any way you want, no matter where you use it. And depending on your usecase, it also means being able to sell access to it.