It is because they simply DO NOT have a scientific, rational, or logical case against the existence of UFOs.
UFOs exist.
Very few Skeptics doubt that there are not Unidentified flying objects. What we doubt and dismiss, based on flimsy evidence, is the alien hypothesis for UFO'S.
WOW! The Universe is estimated at 15 billion or so... and the earth and the Sun about 4-5 billion.
That is irrelevant to the subject of Ufo's