Because there hasn't been an alt-mainstream since the rise of the internet.

Mainstream did survive (e.g. Taylor Swift), but apart from that there hasn't been any of these huge not-quite-mainstream bands for people who don't want to listen to mainstream.

In the 70s, if you didn't want to listen to mainstream pop, you'd listen to one of the handful non-main-mainstream bands your small record store had in stock. That was e.g. AC/DC, Metallica, Deep Purple, that kind of stuff.

But since the rise of the internet, MP3, Spotify and all that, people really have choice. If you want to, you can listen to an obscure little band producing professional quality music from a bedroom somewhere in the mountains of Bolivia. That means, if you don't want mainstream, there's a lot of really-not-mainstream easily accessible, and thus there's not a lot of non-main-mainstream bands that a lot of people actually know.

That's why all the old stuff gets recycled endlessly. Because it's recognizable.