Because he personally witnessed one hovering in the sky for 20 minuets in Berkeley in 1968.
Lots of things were observed hovering in the sky in Berkeley and other nearby locales in 1968.
Lucy, and her diamonds, come to mind.
I'll wait for the article in Science or Nature.
Hmm, Berkeley, '68. Come to think of it, could that one have been a mass hallucination due to a shared batch of acid?