Check out my other comment. It's also that older music was a lot more information-rich, i.e., your brain was processing a lot more stuff, so it was more simulated by it. Music right now is the equivalent of junk food for your brain.
Discussion
I read that. Depends on what music. The complexity in the music is a complex thing :). In terms of the melodies, harmonies and how they evolve, a lot of the pop got super lobotomized as the laptops became the main instruments. But at the same time, a lot of it is heavy on sound design and creating a "vibe".
Also complexity doesn't equal good music. A lot of great music is (deceptivly) simple.
https://www.youtube.com/watch?v=TzIZPZN5K60
Yes, one can argue this Arvo Part piece is not exactly information-poor, but in music, simple is often a good thing that a few can pull off well.
You said it yourself: simple doesn't mean information-poor.
"Good" or "bad" are not the matter here, that's just taste. I can dislike very information-rich music, and it would still be a fact that it is information-rich and that listening to it doesn't have the same effect on people's brains than listening to an information-poor piece of music that's widely liked.