I can understand why you'd get that impression by only looking at the top 40, there's a thriving middle ground of artists that are popular enough to do national/world tours but don't get radio play. Rock and folk are hugely popular (under the "indie" label). The metal scene is thriving. There are young musicians pushing the boundaries of jazz. Electronic music is huge and has more subgenres than anyone can count. The cost of creating and distributing music has fallen massively in the last two decades. More music is being made now than ever before, and small artists are able to reach audiences that would never have been possible just 10-15 years ago.

There's also the fact that it's a lot harder to identify what will be have long-term impact in the moment. To cherry pick a couple examples from those lists, nobody was talking about Nick Drake in 1969. I want to say he sold something like a total of <5000 albums by the time he died. Similarly, Green Day didn't really see widespread success until the 90's. I'm sure there are plenty of currently unknown artists who released their first albums last year who will go on to see mainstream success in the future.

I'd also add that the 2021 list doesn't even seem like a great representation of what was popular last year. It pretty much entirely ignores hip hop which has been one of the most commercially dominant genres behind pop the last few years and produces a huge number of hits.