It was so much easier for the aliens in the old days - you could fly your spaceship around and only a few people might be able to see your ship, and even fewer might think of grabbing a camera to film it. Footage from any film would be so grainy and spotty that it would be tough to figure out if it was real or not, and it would be easy for the authorities to claim that it was fake.
These days, almost everyone has a video camera in their pocket, whether it's a Flip-like camcorder or a camera on their cell phones (although the grainy footage with those cameras still exists, thankfully). Instant upload to YouTube makes it very easy for ordinary citizens to capture footage and show the world.
This technology can also be used to fake alien sightings and footage as well - anyone with a camcorder and some basic special effects knowledge (makeup, etc.) can create a realistic-looking video of either a UFO or an actual alien. Witness this video, which currently has more than 1 million views on YouTube, of a "dead alien" found in Russia, in a "notorious Russian UFO hotspot".
Real? Fake? You be the judge. The fact is, if aliens do exist, it's likely that you'll have seen the footage first on YouTube.
The truth is out there. Maybe.