Plenty of knowledge has been lost; it's just not of the type that Ancient Aliens and the like imagine. Rather, it's of smaller things, like great works of literature or things that could have given us greater knowledge of the past.

For instance, the biggest reason why we know so little of the ancient Minoans is because their scripts, Cretan hieroglyphs and Linear A, are still undeciphered.

There are also numerous lost works of literature, like most of Sappho's poetry, the other six works of the Trojan Cycle, Cleitarchus' History of Alexander, Cato the Elder's Origines, Claudius' history of the Etruscans, the memoirs of Agrippina the Younger, the hypothesized Q document that served as a source for the Gospels of Matthew and Luke, and countless others.

None of these are likely to have contained instructions on how to build a nuclear fusion reactor or whatever, but it's still lost knowledge.