I don't usually venture into these parts of the forums, but I was wondering the same thing a while back when I saw a picture of a huge, jam packed stadium Metallica was playing at sometime during the 80s and I was wondering why I never heard of any modern metal bands doing these things. Any time I hear of a huge sports stadium being filled for a rock band, turns out that rock band is composed of people in their 50s or 60s. Felt strange.
Lot of good insight into the reasons here. Probably a combination of factors including, but not limited to: generational shifts, new music editing software, lack of interest in the Music Industry, and a whole host of other reasons.
The few modern bands I have heard all seem... bland. There seems to be little inspiration both for the playing of instruments and the lyrics. Just take Metallica for instance, Nothing Else Matters (at the video timestamp). That sort of guitar riff still send shivers down my spine.
Someone also noted how easy it is to make "perfectly" played music now, and how hollow it sounds. This is something I have also noticed, though I first noticed it not with Rock, but with classical music a while back when I found these two versions of Bach's Tocata and Fuge:
This version is played by an algorithm, and is there fore played absolutely perfectly. Each note is played for exactly the correct amount of time, and at the right time.
This version is played by Karl Richter, probably one of the best Bach performers the 20th century saw:
Compared to the former, Richter is not playing perfectly. Yet you will be hard pressed to find a finer human player, ever. And that's the rub. I came to realise that people who dedicate their lives to playing an instrument, whether it's classical, rock or other, do not manage to play perfectly, but instead manage to perfect the imperfect. By that I mean they play as good as is humanly possible, but since they still are human they still have these minor imperfections, hold a note slightly too long or short, timing is very slightly off, etc. But that is actually a good thing, as it means they are playing with feeling and we, the listener, can hear and notice these slight imperfections which turns the otherwise "dead" music "alive".
My 2 cents for whatever they're worth.