Lots of people have seen lots of unexplained things.
Seeing something that the mind is not able to interpret is not proof of a Cretaceous species of dinosaur living in Loch Ness, anymore than it is proof of interstellar spacecraft piloted by aliens.
Extraordinary claims require extraordinary proof.
I did not say the only possible explanation is experimental aircraft. In fact I do not think we can chalk up UFOs to one single explanation. I think these hypotheses are also on the table: optical distortion, the human mind misinterpreting optical sensory data, meteorites, as-yet-unknown natural phenomena, drones, experimental aircraft, hoaxes, misidentification, etc.
I do not 100 percent rule out anything, but the claim that space aliens are piloting extraterrestrial spacecraft around our planet on a consistent basis is an extraordinary claim which requires extraordinary proof.