What has happened is that great songs get remembered but bad or average ones are forgotten. To take for example many people would say 1976 what a great year for music. Which is true there were many great songs released that year like Dream On", "Give Up the Funk", "Bohemian Rhapsody", "The Boys Are Back in Town", "Love Rollercoaster", "Evil Woman", "Rock and Roll All Nite, Dirty Deeds, Don't Fear the Reaper, Anarchy in the UK and Hurricane all of these are great songs. That is what they are thinking about what they are not thinking about are other songs of 1976 like Disco Duck, Let her in, Shop Around , Convoy and Afternoon Delight. That is what they are doing it is a variation of nostalgia brought about by listening to their parents music which does not include the worse music of the era.