I would say it’s largely a media construction, rather than a characteristic of the decades themselves. Bearing in mind that when people talk about “the sixties” they’re really talking about the mid-late 1960s and probably only 1966, 1967, 1969. When people talk about “the eighties” they’re probably not thinking about 1987. Because our view of the past is shaped by the media, and the media latches on to certain periods, and this snowballs, and that is where we get our vision of the past. It so happens that there was an explosion of culture in the west in the 1960s, so that decade has had a lot of coverage. Boring old farts from that decade patted themselves on the back, metaphorically speaking, and seduced a new generation with their tales of easy drugs and lots of sex. I think of the 1960s as an overweight old man wearing leather trousers, boasting about how great things were back in the day; and some of it is true, but that was a long time ago, and there’s no need to do subsequent generations down.
Do people think about the 1250s? The 1510s? In my opinion, for most people, every single decade before the 1920s is an anonymous, interchangeable mass of things that happened, just isolated dates and events, and that the only decades people really have a defined view of are (in roughly descending order) the 1960s, the 1980s, the 1970s, the 1990s, and perhaps the 1920s and 1930s. But in the latter case the decade is just James Cagney in a gangster outfit, and the former is flappers. The 1940s as a decade is overshadowed by the war, which exists outside of time.
Yes, we’re all experts on the 1860s. But they - them - they aren’t. As time goes on a decade becomes a set of cultural icons and slogans and then it gradually fades away.
On a tangent, there was a nostalgia boom in the 1970s, and as a consequence I partly associate the 1970s with the 1930s (The Sting, Chinatown, the look of Vogue magazine during that decade) and the 1950s (American Graffiti, Happy Days, Grease), rather than itself. Perhaps by the 1970s the unconscious process of iconographising past decades became conscious, and since then attempts to pigeon-hole decades have been complicated by the fact everybody is trying to “own” the narrative, e.g. Time magazine with its top ten lists of the previous decades, as if by owning the past you could skew the future.