The big problem is that dipping your toes into something via a website (assuming you don't hit an auth wall) is easier than finding then downloading an app.
Besides, a lot of these "web" apps are just things like Facebook or G+ which are no more open to scraping or any of the other supposed benefits of an open web than your average iOS or Android mobile clients are. The web has morphed into an unholy mix of the worst qualities of everything else.