It's like any other phenomina, which have been disproved with advancment of technology.
take spontaneous human combustion, which turns out to be nothing of the sort. Through experiements.
UFOs are the same. Large number is people not understanding what they see. if you live close to an airport, you will occasionally see airplanes which appear to have no blinking lights or, look like they have no wings. But it's just the angle you are looking at them and once they start banking, you see the full shape.
then there is a lot of mistaken identity, either current milliatry aircraft or top secret craft and testing. Just have to look at the sighting of the stealth planes, Sr77 and others. They where all sighted as UFOs and many believed of alien origin.
Or several reportings of a saucer with several smaller circles, funny that America released blue prints which are identical to what people drew. Apparently America never built or tested them.
then natural phenomena. That so far we do not understand.
Things like ball lightning, do Aliens cause earthqaukes, or do earthqaukes cause some natural phenomena. we don't yet understand. i know which one is more likely.
There's also that hill range in Australia, with a road either side that runs parallel. People thought they where being followed by Aliens. Turns out after investigations, that in certain atmospheric conditions, car beams can be reflected/refracted over the hill. As the roads run parallel, it looks like you are being followed. Was in new scientists probably 10-15years. ago. whist I could find the article.