When I see modern media (movies, books, video games) set in the past, it feels like there's a big difference in how people from these time periods act/talk/think. I don't know if this is 'a thing' or if I'm just seeing things, so hear me out.
In the "ancient" category, I think of stuff like Spartacus, HBO's Rome, Xena, and the city-builder games like Caesar and Pharaoh. Doesn't matter if the tone is serious or campy, ancient people seem to be pretty down to earth, they talk normally, they tend to like their vices more...I dunno, they just "feel" like regular people in a different time period. On the flip side, the middle ages feel kind of rigid. Excalibur, Crusader Kings, any Shakespeare movie, A Knight's Tale, Connecticut Yankee in King Arthur's Court, even Game of Thrones to a lesser extent. The poor people are more downtrodden and pious, everyone cares about the lords a lot more, everyone's more pious, and they talk like they're at a RenFair. GoT is kind of the exception in some ways, but even though it's not really medieval, that's the spirit of it, and the peasants have "pious hive-mind" syndrome. I'm pretty sure the fall of Rome didn't flip a switch somewhere that made the actual people act differently, so what's up with all this? Am I even making any sense?
This perception, in the Western world in particular, really goes back to the early Renaissance. Around this time we see the development of scholars using philological analysis - basically analysing the historical development of languages and texts. Scholars, like Petrarch for example, were able to use these techniques to overturn earlier interpretations of Classical Latin and Greek texts.
Why is this important? Because up until this time it wasn't uncommon for earlier scholars to look at the Classics without noting their context. They read the works of Aristotle, Plato, Cicero, etc and analysed the content and philosophy, but a lot less attention was given to the actual study of the context of these writers, their worldview and what the Classical past was like. Part of this problem was erroneous translations, transliterations and re-writings of classical texts which deviated from or misunderstood the original form of the Classical Latin and Ancient Greek texts. Early Renaissance scholars went to painstaking lengths to determine the original forms of these texts and thus learn more about the texts themselves.
When scholars like Petrarch overturned the earlier ways of reading the classics - they went to great lengths to discuss and understand the world of the Ancient Greeks and Romans and in doing so - they tended to lionize them. They emphasised the education, intelligence and eloquence of ancient writers. This is where the perception of an enlightened past comes from. This is where the term 'Dark Age' comes from. Petrarch compared his own time (which he thought lacking) to the Ancient world and thought he was living in a 'Dark Age' by comparison.
The actual reality is very different. If we only considered the works of the most important politicians, the best poets, the greatest artists and the most astute philosophers of a time period - any era would seem like an enlightened Golden Age. One could quite easily cherry pick their way through the Middle Ages, the philosophy, art, the mathematics of cathedrals and revolutionary technologies they invented and easily construct an image of Medieval Europe just as enlightened -if not more - as how Petrarch saw the Ancient Greeks - it is an error of vision.
We see this pattern sometimes replay through different time periods and places. Ming and Qing dynasty scholars, following a school of thought called Kaozheng (search for evidence) denigrated Song dynasty sources, sometimes legitimately, and greatly favoured Han dynasty period works - sometimes to the detriment of fair scholarship.
It is a byproduct of historians admiring the distant past, that the distant past can seem more modern than the more recent past. The perceptions of Renaissance scholars went on to form the basis of Western education about the Classical past, and this has led to a deep-rooted cultural perception of "Classical Enlightenment".