But I do see similaries between some 50s music and some 60s music. Or between some 60s music and some 70s music. Definitely between some 80s music and some 90s music.

I think recently bias is a bigger factor than you're acknowledging. Of course the recent decades are going to sound more similar to one another...but if you compare a song from 2013 to, say, a song from 1959 -- you're going to notice a HUGE difference.