like everything , when you're with your girlfriend fucks all time, but the more time passes unless do you.
bands change in the time (see the end of the movie trainspoting) the world changes and we with him, early Metallica traveled by bus from state to state, now traveled by plane, you understand me. this affects the albums.
Slayer has a family now, before only fun, lots of friends, much distortion, and now only TV news, news paper, etc etc...and it affects an album.
so all the bands, you understand the point.
if you crazy lived to make music based on experiences.
and now live in the comfort of your home, what you expect?