Speaking from my own experience as someone in the the 18-34 demo, many of my friends and friends of friends, as well as myself know and enjoy listening to 80's pop, rock, etc. as much as they enjoy new music/music we grew up with, Some reasons I find consistently: exposure from our parents (many of which grew up in the 70s and 80s), and exposure from pop culture and media (look at Stranger Things, even popular movies from the 80's). Maybe there is some incidental/non-intentional listening, but there is definitely some intentional listening amongst people in this demo.