They're nostalgic for the pop culture version of the 80s, not actual life in that period.

Growing up, there were still tons of reruns of 80s shows on TV in addition to 80s movies being very common in late night movie slots. The entertainment and pop cultural landscape also seemed pretty familiar to a millennial growing up in the late 90's and 2000's- many of the same franchises, movies, actors, artists and bands I liked originated in that period and you also had things like home computers, video game consoles, table top RPG's and so on, just cool old school versions. I've also always found the look and sound of 80s music, fashion and movies to be cool (again, probably because I was exposed to it as a kid) whereas the 60s and 70s just seemed like dad decades to me.

The very shallow ethos of the era has also somehow always had a weird appeal to me, it's all about flashy colourful disposable fun whereas in the 60s and 70s you have a bunch of guys with horrible hair and even more horrible mustaches taking LSD and being angry about the Vietnam War or whatever