One possible answer is that most of our observational system are very specialised for a target and they ignore any other objects as noise. To record UFOs – if they do exist and can be recorded – one need a net of telescopes with very wide angles or many airplanes with different sensors. The last thing is similar to a squadrons of military aircrafts.
Certainly it is possible that we are not equipped for recording UFOs, but then, given the amount of sightings and that we have had grainy images for decades, the technology should have improved by now to have something more definite.
Good point. Actually, I think that all almost early time “saucer’s photos” are home-made fakes.