Consider this situation: You drop a box with 6 months worth of backups down the stairs. If these are tapes in their cases, you pick the box back up and continue on your way. If they are disks, you just lost 6 months worth of backups and you'll probably be job hunting.
It's common practice for couriers to take tapes offsite daily and then return them later. Where I work, every day a courier comes and picks up the daily tapes and drops off the tapes from 1 month ago. These tapes typically stay in service for 2 years or more. That means at least 24 trips in and out of our building alone in their lifetime, stashed in the back of a truck, stacked on a shelf, etc. Tapes handle the jostling and jolting of these trips much better than disk.
Also, you can commonly get a 4U library that can hold 30 LTO5 tapes. LTO5 tapes can hold 1.5TB on them. That's 40.5TB of raw backup storage in 4U. You'd be hard pressed to do this with disks.
RDX disks do exist, which are basically an attempt to directly replace tape with disk, but they never really caught on and are generally more expensive than comparable tape with less support because of the adoption rate.
I should mention that a lot of times disk-to-disk archival makes a lot of sense for a lot of businesses. Nowadays you see a hybrid situation where you'll go disk-to-disk with dedupe for normal backups and you write the whole dedupe pool off to tape once a week/month/whatever.
The key to doing this and not losing your job in a disaster is to make sure that you have off-site backups available. For some places, that means putting something like an Exagrid at a colo'd location or dumping your backups on a SAN volume and using that SAN's replication technology to replicate your backups offsite. Whatever route you choose, you need to make sure that off-site storage happens. All of the backups in the world won't mean a thing if your building burns down.