CPUs used to be too slow, were without AES-NI, and running both Triple-DES encryption and LZ compression on say a Pentium at tape's data speed was a challenge. So people wanted encryption plus compression done within the drive. I imagine the IBM ppc CPU has accelerators for both. In that scheme backup software also isn't bothered to implement it.
If you pull encryption from drive into CPU, then drive compression will no longer be able to compress the fully random stream at all. Nil. Then you'd also have to run compression on CPU, then encryption on CPU, in that order, and send that stream to tape drive.
With advent of Haswell, AES-NI, and later modern compression methods like LZO, LZ4 or ZSTD, backup software can do everything in software.
I have never implemented TS3x00 encryption on the unit. If tapes are not readable in another library, this would be a deal breaker. Better take your favorite book, extract 10 words from your favorite passage, run that through SHA2-512 and that will be your key, stored in the backup software. If you lose everything but the tapes, all you need is ebay for a drive and a book shop.
For safe keeping (pun intended), I like mid-1990s vintage class IV safes from bank branch closures at 1.5 to 2.5 tonnes per safe.