It is a question of failure mode. If your hard drive motor dies, if the heads fail, if the electronics fail, if the sealing fail (in some cases it can be repaired by specialists but that's not easy and much more expensive than a tape reader)... lots of things can go wrong that are not related with the media itself. The advantage of tape is that the device that reads it is separate from the mechanically simple tape. It comes at a cost but that's for hedging against a different risk.