Astronomers do not spot aircraft in their telescope. This does not imply that there are no aircraft. Telescopes are just not built to spot near objects, there are built to observe far objects. By the way, astronomers do see UFOs just as often as other persons, but not through their telescope, rather with their naked eyes.
First, UFOs are indeed spotted on radar.
You need some information on this topic. Civilian radars usually detect only known aircraft, they carry a device called a transponder that emits a identifying signal, and civilian radars only show airfacts that emit this identification, per default. Military radars show all aircraft of course, known or unknown. This means that when a civilian pilot reports a ufo sighting, the civilian radar controller generally asks the closest military radar controller to check if there is a radar signal.
There are a lot of cases in which military radars confirmed the traces of a ufo flyby on their radar, or even sent fighter aircraft to chase UFOs, both ground and aircraft radar being used for that, and radar traces being published as proof of the material existence of the UFOs. The Belgium flap in 1990 is a good example of such an event.
All social categories, all cultures, all professions are equally represented among the UFOs witnesses.
On the contrary, their are UFOs reports existing throughout the human history. Old text such as the antique indian Vedas, old paintings and drawings mention them, often with high accuracy and details. Refer to the historical page of this site for more information.
There was only one wide scale objective scientific investigation conducted on UFOs, in France, by a state service, GEPAN/SEPRA. They have now concluded than in some rare cases, there are sufficient physical evidence that some UFOs are flying machines driven by an unknown intelligence and capable of flight performances that human technology is nowhere near to match.
Skeptics had this idea and quickly started statistical study to prove it. But the result were negative: UFOs appearances do not increase after movies with extraterrestrial themes are shown, no link whatsoever was found between media events and UFO flaps.