Because applications also provide very shallow functionality -- and especially most of the ones I see asking for notifications (makes sense too: marketers and spammers and the first to jump on such features).
Also because a user can always go to the settings of the application and explicitly enable it. That's why there are settings.
If they don't like settings pages, and think users would not find them, they could also have an "enable notifications" link/button somewhere prominent in the header/footer etc for the user to click, instead of directly asking users.
As for non-tech savvy people, notifications and popups just confuse most of them, and they blindly tend to click to dismiss them, either yes or no, often without even reading the message carefully (or at all). That's something that has been hammered on by UX experts since the dawn of time.