File compression requires additional storage, memory and processing power. Why bother if the tape appliance already handle it ? Data is unusable in compressed format and is hard to deduplicate. Also, often, there is already compression at the storage array level but the data is decompressed when read.
What data is unusable in compressed format?
Images, videos, music are compressed. RAWs from my camera are compressed too. Even log files tend to be compressed.
What else do people store that would amount to multiples of 30TB and not already have some form of compression?
Databases and their transaction logs, operating system files just to name a couple. Tape backups are not for home-labbers.
LTO is normally used with high-end backup software like Commvault that compresses and dedupes backups before writing them to tape.