Perhaps having grown up in the '80s, my take on the ebb and flow of styles between the '80s and now is different from those who saw so many quick changes during the '60s and '70s. I can tell a difference between '80s, '90s and '00s styles.
But the problem is that starting in the late '90s, people started creating new fashion statements by fusing modern dress and stylized period dress. Streamlining the more evocative and radical looks of the '70s (and late '60s), the bell bottoms made a subdued return as did tie-dye in a limited quantity, sandels, long hair, lots of accessories and so forth. Those styles were mixed with "business casual" to create work-appropriate dress (that still hasn't filtered successfully into many work places that still don't permit jeans, but allow cargo pants because they are khaki colored, but that's a digression).
Then, as the '00s played on, the styles of the '80s started to infuse themselves into these new trends further blurring the line. While they picked up some trends (like wide-rimmed black-framed glasses) from earlier periods, things like modified legwarmers and high-top sneakers (and now boat shoes) started making new appearances, and even the "grunge" styles of the '90s were blended together.
So, modern attire seems reminiscent of the dress of the 1970s, 1980s and 1990s where the distinction in styles from prior decades was much more noticeable. Thus, it's easy to muddle what is modern dress for the '10s and what's modern dress for the 1970s/1980s/1990s because of just how much has been heavily borrowed or is heavily influencing recent trends.
So, my guess is because there is no distinctive style (homogenized, even) that can be achieved to make it really feel as if from another era, the contemporary category is merely reflecting modern trends even when borrowed heavily and the influences of those modern trends. The only thing these days that really seems to distinguish the "modern" period from the 1970s/1980s/1990s is hairstyles (though there's even some blurring in those regards).
So, until the modern style is appreciatively different from the 1980s/1990s periods, I think we'll continue to see many of those films categorized as such.
"Any society that would give up a little liberty to gain a little security will deserve neither and lose both." - Benjamin Franklin