There are plenty of good (and bad) bands and songs that both you and I haven't heard of.
The heap of content ∞
Time rolls on, and ever more gets added to humanity's heap of accomplishments. These aren't just the giants whose shoulders we stand on, but every little grain of sand that together makes our collective mountain.
Not many things are interesting enough for commercial interests to keep them even slightly afloat.. like a song. Afloat enough for you to have heard of it.
Even the most popular songs will become copyrighted material that only exists to be licensed as cover songs. Perhaps they will be released in compilations, or re-published in movie soundtracks. Maybe then you will have heard of it, or more likely you will have heard of some "new" version of it.
Indie content ∞
Now imagine indie bands, never published. They are born, create music, and disband.. and you'll never know of them. You could be passionate about their genre, know of every signed band and every one of their songs by heart, and have never heard of the majority of that genre's artists.
They do covers of bands you love, even write their own music, but you've never heard a single song.
By the time a new "scene" comes to your attention, it has been alive in small communities at bars, clubs and garages you'll never visit. It isn't a breakthrough band which makes a new sound, but a huge collective from which that band was picked by commercial interests.
Take a genre. Rock and roll, jazz, hip-hop, rap, grunge. Anything you like. Try to go back to its roots, and you will fail. Musical historians, even now, can't stitch together the pieces that made a genre become what it is.
I know of efforts from years ago who wanted to learn the roots of grunge. It was post-Nirvana, which disbanded in 1994. They went to Seattle. They interviewed people and visited all the relevant places. Though they found the occasional person and band from back then, the majority of the scene had shifted, moved or evaporated into mere stories.
Living memory can capture some of what a scene is, and can record a piece of it, but that knowledge inevitably fades.
Not just songs and bands ∞
It isn't just songs and bands, but everything. A piece of software, a piece of hardware, a toy, a business.
Things get buried under our collective mountain of accomplishments. Only some things are dug up to the surface again, where some things remain of interest to hobbyists or specialists and some things are entirely forgotten.