Because someone at Apple screwed up. Might be fixed eventually, or not.
There was this bug where, every app that asked you if you wish to allow notification, and you say no, it gets put in your notification area anyway. So every time you installed a new app and it asked this, you have to go into settings and manually turn off the notifications. As far as I know, this had been the case since iOS 4. Finally, it's fixed in iOS 8.
So let's hope this gets fixed by iOS 12, or if they have pity on us and work a little harder, by iOS 11.