UFO's are a subject that the government and military can't really ignore as so many people claim to see them. I'm sure most people in government and the military find the whole subject very tedious and wish it would go away, but they can't ignore it.
It's been shown that people claiming UFO sightings and alien contacts aren't a new phenomenon. It seems there's always been a section of society who feel the need to claim sightings or contact with whatever phenomena are most mysterious at the time. In the past, such people reported visions and contacts with angels, heavenly signs in the sky, apparitions and communication with saints etc. Today, the most mysterious subject for the average person is space, the universe and possible alien life forms. No one claims they've seen angels today, signs in the sky or a saint has appeared to them - but many now claim sightings or contact with aliens.
Secondly of course, many people often report perfectly explainable objects which they don't understand or recognise. The most commonly reported UFO sightings actually prove to be - the moon (seen in different atmospheric conditions)! The second common-most are sightings of Venus. Many more are aircraft.