During a remarkable congressional hearing, former American intelligence official David Grusch revealed that the US government conducted a "multi-decade" programme aimed at collecting and reverse-engineering crashed unidentified flying objects (UFOs).
Grusch, who was in charge of investigating unexplained anomalous phenomena within a US Department of Defence agency until 2023, spoke before the House oversight committee in Washington, shedding light on the issue of alien life and technology, as reported by the Guardian.
Thats the point of the phrase. The idea is so unbelievable that it needs to be personally verified before it can be taken seriously…
Not to mention, you can see something and still not ‘know’ - stage magic, for one.
See also: ‘it has to be seen to be believed’; ‘i cant believe my eyes’; the word ‘unbelievable’
No end of useless pedantry these days