In cases like these, you're dealing with what we call survivor bias. This is the notion that not everything gets preserved, or at least preserved well enough to be easily found and held up as an example.

This is exceptionally true of archaeology and periods before the advent of particular types of communication. It's difficult to study non-mainstream music, for example, before the advent of pressed recordings. It's hard to examine early film because so many moving pictures used nitrate cellulose, which decays rapidly.

Let me give you a particular example. You're probably familiar with the music of the 1960s. You've heard it played on "oldies" radio stations, your parents' (or grandparents') vinyl, and in car commercials. But bands like the Beach Boys or the Beatles are only the mainstream. They certainly reached millions of people, but if you believe in the long tail phenomenon, millions of people worldwide listened to music that never hit it big or was preserved, even though it may have been historically significant.

In the mid-1960s, a group of disaffected American soldiers with musical talent formed a group that toured Germany. They were called The Monks.

"Instead of love (they played) hate and negation," this 2009 NPR retrospective said, and that's as apt a summary as I can make myself.

The Monks, with the benefit of our hindsight, are clearly an ancestor of 1980s punk, but they were an abject commercial failure. No one wanted to listen to them in the 1960s. Today, they're preserved but obscure ─ more notable for what came later than anything they did themselves. They're also used by music fans who like to flaunt obscure knowledge.

In any event, that's just an example of what I'm talking about. Survivor bias is something that you have to look out for as a trained historian. When analyzing books, popular music, sculpture and the like, you want to look for publishing statistics and reviews, things that'll tell you exactly how widespread a thing was and how it was accepted at the time.

Something else to consider in addition to /u/The_Alaskan's excellent points about survivor bias and the long tail phenomenon is that perspective is an amazing thing. Being inundated with music and fashion and art and advertising of today makes it difficult to stand back and get a good look at the cohesive whole.

A lot of people can tell "Victorian" from "1920s" and "Marie Antoinette", and think that these are monolithic eras defined by a few particular characteristics. With more experience and research, it becomes possible to define the style of a decade. By putting in the effort to look year by year, you learn that styles aren't bounded by decades. From the perspective of someone who doesn't care much about fashion history, there's negligible difference between 1861 and 1865 and [1868] (https://digitalcollections.nypl.org/items/510d47e0-e157-a3d9-e040-e00a18064a99), but from my perspective these are quite distinct and not confusable. There are lots of subtle markers to distinguish sub-sub-periods in dress and which make it impossible for an informed observer to describe the "defining style" of a broad period of time - whether the observer is informed through extensive research, or through lived experience.

Does that make sense?

I would recommend you go to /r/arthistory for answers if no one posts here.

I agree with what /u/The_Alaskan and /u/chocolatepot say about survivor bias and the loss of fine-grained distinction with historical distance, but there are art historical factors involved as well. The first question is: which art, from which period and where? You refer to the medieval period through the present, and your period designations suggest that you are only thinking about Western art. In that period and place, art historians more or less agree on broad period distinctions (medieval, renaissance) as umbrella terms within which finer distinctions can be made according to period, place, genre, etc.. Then you have terms that more explicitly signify style even as they indicate period (e.g. mannerist more or less the 16th century, baroque more or less the 17th-1st half of 18th century). But again if you look more closely you will see enormous and often contradictory variety of style by author, date, location, genre, medium, etc. (e.g. Poussin's neoclassicism and Rubens' baroqueness both falling under the general category of baroque)--this confirms what /u/chocolatepot says. In Europe this state of affairs continues until the second half of the 19th century when the relationship between art and the state fractures and you have a multiplication of stylistic options and movements--in the 20th century this gets grouped under the umbrella term 'modernism'. But what you're taking about today--the lack of a defining style--is different, a result of the exhaustion of the avant-garde questions that motivated the splinter groups of modernism, and one much more important thing: globalization. The rapid circulation of electronic images, the globalized art world, the attempt by art historical institutions to grasp the multiplicity of world cultures have broken down that neat historical lineage that you refer to (medieval, renaissance, up to the present) that is centered in europe. So while survivor bias and the effects of historical distance are real, the sense of a dominant style/way of working/artistic question that is the result of a more or less unified (european) culture no longer obtains--a result of institutional critique (typically understood to have begun with postmodernism) but also of geopolitical and cultural realities of the last 40 or so years.