Because 2000-2010, and 2010-2020 had/have no "good names" we started to associate trends and things with Gens instead of the decades. (Or into other categories)

E.g. the Mullet is an "80s haircut". A Xenial might have rocked it in 1st grade to be like his GenX cousin, who rocked it in high-school, who was emulating boomer uncle Mike who rocked it to pick up chick's at the bar.   But this all happend in the year 1987 so we consider mullets an "80s cut".

When that "broccoli cut" became popular we associate with the Gen who mostly wears/wore it instead of the era it became popular because "two thousands teens cut" doesn't roll of the tounge like "the 80s" 

Think music. Cindy Lauper, Motley crew and Duran Duran are "80s music" , then divided by sub genres like hair metal, rock, pop.

Katy Perry, Maroon 5, 5 finger death punch are labeled subgenras like pop and heavy metal 1st, because "twenty tens" or "twenty hundreds" music doesn't make a "good lable". We aren't reneforcing/ defining the idea of what the 2010s were like.

(The guy from Adam ruins everything has a YouTube video on this subject with more examples, better explanation.  it makes sense to me)