That's what happened, how do you think we went from muskets and bartering to lawsuits and online shopping? Just compare the US of present day to US of 1819, 200 years today. Take a good long look, and tell me we haven't forgotten basically everything we knew then and traded for a more convenient system of doing things? This isn't just the US either, but the US did it the fastest, going from 0->superpower in two centuries. But current age, after the wars were all said and done, a lot of people look at US as lazy, and they would be correct comparing to old standards, but we have improved technologies, longer lifespans, it's honestly rare that people are starving even if they are homeless. Processes change. You could also consider it(going back to manga) a conspiracy to discourage future evil. Everyone "de-escalating" their fighting prowess to prevent war amongst humans since the demons were no longer an issue. The bottom line is, a culture can change in less than 20 years, it's laughable to think 200 couldn't change it.
@Shakur OH, and the dark ages would be good example, loads of lost info because of text->text replication, printing books wasn't a process for a long while, and based on the look of their technology, it would make sense for them to cut corners a lot.