Maybe because I don't want a bloody app for every site that I browse? Apps of that sort are just dumb. And huge security risks.
I mean the UI is undeniably smoother, and they can seamlessly hook into the OS notifications systems and better multitasking (for example I see separate entries for each native app in the OS's task switcher, but have to go through the extra step of switching into a browser then its tabs for webapps) and energy saving and everything else.
And there's the problem. I don't use any of those four sites regularly, but I have visited all of them. Hyperlinks provide for that, and they (a) don't exist or at best would be awkward in a native app (not that webapps handle them well to begin with, though all of the above do allow for them at least) and (b) work between apps, platforms, and what-have-you. If I got a link to some image, <160 character sentence, comment thread, or song and was prompted to download an app, I would probably not view that content instead.