At the end of the day whether or not the source data is in a proprietary format or not, it's being rendered into HTML in the case of web apps. The only difference is whether that happens on the client or server side. It's trivial to parse in either case. In fact I'd argue it's usually easier to dig up a JSON end point in the case of a web app which is far more parseable than HTML.

I don't think we can point to this reason to explain the rise of web app's.