Lots of tape drives offer built-in compression. The drive firmware does it. On AIT3 drives you can enable or disable it. Compression may have been a big deal

Lots of tape drives offer built-in compression. The drive firmware does it. On AIT3 drives you can enable or disable it. Compression may have been a big deal in the ages of sub-1Ghz single core systems but not anymore. Especially since you ought to be encrypting before writing anyway. Why does it continue? Probably so they can double the capacity that they put on the tape label, which is always the compressed capacity and not the native or real capacity. Plus backward compatible with such encoded tapes.