There’s a lot of history around that split, and the motivation for HTML/CSS was about separating presentation from the content in many ways. For another example, once upon a time a lot of chat services ran over XMPP, and you could chat with a Facebook friend from your Google Hangouts account. Of course, both Google and Facebook stopped supporting it pretty quickly to focus on the “experience” of their own chat software.
The thing is that there is very little money to be made selling content, and a lot to be made controlling the presentation. So everyone focuses on the latter, and that’s why we live in a software world of walled gardens that work very hard to not let you see your own data.
There is some EU legislation proposal that may make things a bit better (social network interop), but given the outsized capital and power of internet companies i’m not holding my breath.