Because I'm not interested in downloading an app for every site or social network. If viewing a G+ post without having a dismal experience is going to require a dedicated app, then I'm simply not going to use G+ at all. That represents a really lame barrier to entry for alot of prospective G+ friends, so why bother? Particularly in the case of a social network, I expect the user experience on the plain old web, mobile included, to be a first-class concern.

For all the UI's clean, minimalistic pretenses, it's ultimately a hog underneath. If there is no adult supervision on the G+ team to prevent this mess happening, because after all people should just download the app or something, then again, I'm inclined to steer well clear of it.

Apps sometimes appear to be reinventing bookmarks.

Not the case with the G+ Android application: instant photo upload, background polling, offline content caching, video Hangouts. It's certainly not a browser page facade.