Many thousands of people every year report that they are seeing UFOs and are being abducted by aliens for unknown purposes, and it costs the government of the world millions in recording and investigating these claims. However the vast majority of the time things are reported there is a rational explanation, and the UFO is often nothing more than a balloon or military plane.
If aliens indeed have visited the planet, then they would have to have technology far in advance of our own, when you consider that it would take us 30,000 years to reach the nearest star to us with our most advanced space shuttles. Bearing this in mind surely any aliens who were capable of this would also be capable of hiding their presence from us and not attracting as much attention as they seem to. Not only this but they would likely have enough of an understanding about human physiology to erase the memories of anyone that they abducted.
Aliens as advanced as this would likely only need a very small amount of people to experiment on in order to understand the species, and could of course make their own humans in laboratories if they so wished. Added to this is the fact that comparing humans to aliens this advanced is similar to comparing ants to humans. Their attitudes to such a primitive form of life as humans would probably not be as compassionate as hiding their existence from people and only appearing in very blurry photos.
Because of this it seems unlikely at the very least that the UFOs people are seeing every day have come from anywhere other than somewhere on Earth. Many governments of the world have experimental planes and aircraft that might appear to be alien in nature, and these are often seen by people. Of course the fact that they officially don't exist and that the military often refuses to confirm or deny whether a sighting was something of theirs only adds fuel to the idea of aliens.
A good example of this is that during the second world war, the Nazis were researching and developing flying saucers, which had up o that point only been seen in movies. Before their demise and the end of the world war, they had gotten as far a long as getting these strange vehicles to fly, and were working on equipping them militarily. If you now also realize that this all happened 80 years ago, then the kinds of top secret aircraft that could be flying about in the skies today could be truly beyond belief.
It is also worth noting that the amount of sightings of supposed UFOs is a lot higher when there are military bases in the area, particularly those which might be researching new aircraft. Surely aliens wouldn't need to waste their time spying on military hardware thousands of years behind their current capabilities. However by denying responsibility for these sightings as their own aircraft, the military can ensure that the only people watching the skies will be anoraks and conspiracy enthusiasts who are largely ignored by the rest of the population.
It has also been noted that many encounters that people have with aliens and abduction experiences are merely side effects of sleep paralysis, which is also where the idea of trolls, vampires and succubae are thought to come from. These waking nightmares often seem very vivid, and the person is awake yet paralyzed, which they often take to be something that aliens have done. So as far as abductions in relation to UFO sightings go, they can largely be disproved as being real.
All of this points to the fact that either people make up UFO sightings to get attention, or are mistaken in what they are seeing. Military aircraft, as well as weather balloons and the like could often be confused with a UFO, particularly if they aren't acknowledged on any level by the authorities. As well as which, by the very nature of the media, a story about a UFO is always a lot more newsworthy and exciting than something dispelling the idea and identifying sightings as something rather more mundane.