Because they see that the image looks sharp and they think they took a great picture, when in reality, there's much more in order to make a photograph look great, but many have never taken basic classes of photography or video, so they don't know.
Because people think if they can take a photo, that makes them photographers. Especially when they throw a filter on it to make it "artsy". Cause apparently that's all that real photographers do, according to some people I know. As a photographer myself, I rarely take artistic looking photos with my phone. I reserve my phone's camera for photos of my dog, friends, and random things I find funny. I have a professional camera for my profession. Not just an iPhone, no matter how good of photos those things can take nowadays.
Just because they can take good pictures doesn't mean they believe they're photographers. I am an ASPIRING photographer, and I had an Iphone. The camera was a lot better than the REAL camera I had so I used my phone more often. Doesn't mean I think I'm a photographer now.
To me, 'Photography' will always be film, and a dark room, and the 'ART' of the image, in Monochrome. This modern digital sht has destroyed the 'Art' of making the image!! Taking the shot is just the start. The rest is in the Dark Room, and dodging/burning, to make the RIGHT image, to express what you were feeling, taking the shot!!