Many years of interest in science

Well, first let me present two equations.

It is clear to me there should be a few intelligent species out there. Even pessimistic numbers gives us one or 2 on our galaxy.

*from XKCD The Flake Equation

The trouble with your question is the term ‘credible witness.’

A credible witness is not proof or reliable evidence. What the world needs is proof and evidence of aliens visiting us, not a few witness. We have had decades of ‘credible witnesses’ but still not a shred of physical proof.

But I’m still waiting for concrete evidence.

Keep in mind a UFO is an unidentified flying object. If I see a blimp or a weather balloon or a sundog or the planet Venus or a satellite flare and don’t know what it is, that’s a UFO. Simply reporting something strange doesn’t make it aliens, even if your report is reliable and credible.

And, crucially, the people who are not reporting UFOs are amateur astronomers. Consider that amateur astronomy is a hobby that often means most clear nights someone is outside staring at the sky. If UFOs are something truly new, and not uncommon sky stuff that people see, then amateur astronomers (not to mention the sky surveys professional astronomers use to look for asteroids and comets and supernovae) should be reporting them in droves. If UFOs are just ‘see something unusual and can’t ID it’, then the people who do learn about satellite flares and stuff won’t report them as ‘unidentified’. It’s a neat little test to see if UFOs are something new or just reports of ‘weird things’ that people more comfortable with the sky can identify, and so far, UFOs are proving to be the ‘unusual but explainable without aliens’.

It does seem that planets are common (though we’re not in a position to say planets with life are common, since we only have one confirmed), but distances between stars are huge. Our fastest current spacecraft would take about 75,000 years to get to the nearest star besides the Sun, and that is a robotic probe (Voyager 1). Getting a robot to the nearest other star in a human lifetime would be a major engineering achievement and incredibly expensive. Extraordinary claims require extraordinary evidence, and so far UFO reports that are substantial enough to be investigated can be explained by a mix of natural phenomena, human-created phenomena, and observer bias.