Well I ask you, by what standards are we supposed to measure advancement? Is there an end state of all civilizations that we can objectively place each and every society that has ever existed on? Are we supposed to measure advancement by the development of certain technologies or political structures? Seeing as real life has no version of Civilization's tech trees that seems a bit hard truth be told. Are we supposed to measure the worth of each society by our own modern standards of what constitutes advancement? How do we measure peace? Do the incessant wars of Greek city states in the 5th century BC not count as a more warlike time than the middle ages? Does Roman rule really constitute "peace" when it came alongside the genocide of local peoples?

These questions might seem a bit pedantic but they are worth reflecting on for a bit.

Today, and especially on the internet in places like Reddit, the idea of what constitutes advancement is pretty simple, progress towards modern society in the west, complete with democratic political structures, secularism, scientific advancement, and so on. Societies are then judged on where they fall on that level of development. This is what historians call teleology, or the assumption that history has an endpoint and that human societies are developing towards a defined end goal.

The only problem with this, ignoring all of the inherent assumptions of cultural superiority and often racism that this point of view carries (which if you are curious about check out the latest episode of the AskHistorians podcast), is that it assumes that societies develop on a single way forwards towards a single end point. Progress might be made or lost, but the end goal remains the same. We should try and avoid thinking this way about past societies. Cultures and societies are not inherently more advanced because they have developed specific methods of metallurgy, or governmental structures that enable popular participation in government, or a means to delegate all the unpleasant labor of the world out of sight. These are all features of a society, worth of interest and study, but they do not tell us anything about the advancement or worth of that society.

Now to narrow in more on your specific question. Why do "we", ie the modern western world, view the Middle Ages as more primitive than the "Classical" Civilizations that went before? It has precious little to do with any actual regression of society between the years 500-1500 to be sure. Societies were every bit as vibrant as always with new forms of artistic expression, literature, technological developments, political strife, and so on. People did not stop writing poetry or philosophy, nor did they cease creating wonderful works of art, and the Middle Ages did not invent warfare, religious strife, pestilence, and abusive government, but why does there remain a lingering perception among the western world regarding the supposed barbarism and primitivism of the Middle Ages in contrast to the Classical world?

In short, because one poet was filled with the same angst about his own generation as many of us were when confronted when Justin Bieber first hit the airwaves. Every since Petrarch complained about how terrible the world he lived in was it has been rather fashionable to hate on the Middle Ages specifically as a time of barbarism, ignorance, superstition, and regression. This mantle was then seized by a certain number of thinkers over the centuries and has formed the basis of western understanding of the Medieval period ever since. You see it in media as diverse as Game of Thrones, Cosmos, Total War games, and so on. The Medieval period is an otherized time in history, when society was in the grip of stupid nobles, violent knights, and above all the biggest of all, the big bads on Reddit, The Church. Indeed the Medieval world is largely what we define the modern world against, hence the word even becoming a by-word for barbarism and cruelty when in reality it was no better or worse than any other time in human history.

If you're more interested in specific example of how life changed over the course of the Middle Ages from the Classical world, let me know and I can either answer or point you in the right direction.