Tape does not operate in an hermetically sealed unit protected from everything, though, that's the point. That's why.
A speck of dust would *destroy* a hard disk platter to make it unreadable (and you can't crack it open to blow it off and carry on even if that worked!). A speck of dust on a tape will be cleaned off by various brushes and be lost in the data's error correction anyway.
Tape has its own problems though - stretching, temperature expansion, and all sorts of other nasties that *don't* affect disks.
Which is why tape should never be your ONLY backup. Hell, stick it in the corporate backup scheme to backup to external USB hard drive WHILE spooling to tape. Because that hard drive won't give you 100% complete backup reliability, but it can be taken off-site too, replaced easily and cheaply, encrypted just the same, but will survive different *types* of disasters and restore more quickly. Hell, I've seen hard drives that have been submerged brought back to life. I don't think tape would quite stand the same treatment. And with backup the biggest word is REDUNDANCY - redundant backups on redundant media with redundant hardware (no point spending £10,000 on a tape backup system if you only have one tape drive and the place floods even if the off-site tapes are intact) with redundant methodologies in redundant locations.