Most UFO sightings are so fleeting that if you don't just happen to have your camera in hand and at the ready, you probably won't get the shot before the thing disappears.
A lot of footage of UFOs is captured by people who are in the process of filming something else when a UFO comes into their field of view. Sometimes folks don't spot the UFOs in their footage until they review it later.
Also, cell phone cameras have pretty bad resolution on average, and do a poor job of capturing detail even close up.
Countless people have claimed to see ETs. There should be time to take a snap of one short of mind control. But like I said, they might be getting more cautious as tech increases [especially when visiting Louis Farakhan].