Why did my backup folder with large amounts of repeated data compress so poorly?

I have a folder with around seventy subfolders, each containing a few tarballs which are nightly backups of a few directories (the largest being /home) from an old Raspberry Pi. Each is a full backup; they are not incremental. These tarballs are not compressed; they are just regular .tar archives. (They were originally compressed with bzip2, but I have decompressed all of them.) This folder totals 49 GiB according to du -h. I compressed this entire folder into a tar archive compressed with zstd. However, the final archive is 32 GiB, not much smaller than the original. Why is this the case, considering that the vast majority of the data should be common among several files, since I obviously was not replacing every file every day?