Just a theory, but given how much fracturing/variation of music tastes people have developed in the last 50 years, I'm not sure there's enough of an audience for any of those mega bands to get to that level anymore (except for a few pop artists here and there).

I also suspect discoverability is a lot more of a challenge now too given the saturation of content out there and the small market. I've discovered some excellent bands (for my musical tastes of course, which are Rock & Metal) that I can't believe aren't bigger. These Four Walls and Thousand Needles In Red come to mind. Like Thieves is pretty decent too, nice music to put in the background on a drive through the canyon or while working. All of those bands are on Bandcamp btw.