@Hell_Satan
Yeah that can be a reason but back then it was much harder to record down and keep track of information back then, all they had paper or stone to etch sure but back then it wasn't cheap for paper. Not to mention there was no printer.
This however is different, we know from the first chapter the year before the fall was 2060, they even had futuristic technology to create something like Leo. Thus I find it hard to believe they lost all that information to war and regressed that much not to mention there still exists bunch of 2060 ruins and somehow impossibly no one dared to excavate and study the year 2060 for their technology.
Humans back than were seperated worse by social class back then, peasants for the most part had no access to free education. However most humans in modern society now do know of a lot of the world and how technology works and develops. We have the base knowledge of it.
I find it hard to believe not a single human decided to continue passing down knowledge or easily wrote it down in 2060 and keep that knowledge existent past 2060. There exists many people in Leo's world who can't use magic so them purely staving off of reliance on magic is weird too.
Probably a conspiracy plot that forced humans to forget about modern knowledge other than just Leo's explanation humans just eventually relied on magic more. Maybe Leo (seeing as hes at least 3000 years old) made technology regressed far back so no one makes more beings like him lol. But that's just me spouting random nonsense.