Because he knows they’re deceptive, masquerading demons.
https://www.bbc.com/news/world-middle-east-48648788
https://www.dailymail.co.uk/news/article-7144893/Trump-says-NOT-convinced-UFOs-real.html
https://www.inforum.com/news/nation/2708912-Canadian-indicted-in-case-alleging-transfer-of-US-Navy-vessel-design-details-to-China