They want fame, notoriety and followers - that way lies a big ego boost and advertising revenue. You see quite a few, er, augmented, pictures these days. Some more subtle than this.

I'm only surprised its not the Zetans coming in off the shoulder of Orion at last. :)

That's a really good page. Unfortunately fake images like this are common. At least this one was blatant, some are more subtle and harder to identify for what they are. A good maxim is that if you see any astro image that shows something no one else is showing from someone you have never heard of, be suspicious and dig around a bit to get some idea of the authenticity. It is a shame that social media is overrun by this kind of stuff. A faked comet picture is pretty minor compared to some of the other, sometimes very convincing, fake messages out there.