Plenty of uncompressed tarballs exist. In fact, if the things I’m archiving are already compressed (e.g. JPEGs), I reach for an uncompressed tarball as my first choice (with my second choice being a macOS .sparsebundle — very nice for network-mounting in macOS, and storable on pretty much anything, but not exactly great if you want to open it on any other OS.)

If we had a random-access file system loopback-image standard (open standard for both the file system and the loopback image container format), maybe we wouldn’t see so many tarballs. But there is no such format.

As for “why archive things at all, instead of just rsyncing a million little files and directories over to your NAS” — because one takes five minutes, and the other eight hours, due to inode creation and per-file-stream ramp-up time.