At least HDDs are fast, cheap, don't have massive up-front investments. You can take a couple of HDDs, RAID them together for reliability and you can still write and read data at humane speeds.
A RAID array isn't a backup. Your data is still live on it, an "rm -rf" away from destruction. HDDs fare poorly when shelved for a long time, RAID are even worse in that regard. OTOH a tape on a shelf is perfectly safe for 5 to 10 years or more (I recently restored 1998/99 sensitive data from LTO2 tapes written in 2003).
Unless you store your tapes right next to your cache of neodymium magnets, that should be pretty easy. Writing to a tape is very deliberate generally. Much more than writing to disks.
With current software and hardware RAID solutions, we also have massive up-front cost. We can't buy drives as needed and add them to the system one at a time.
I wish it was easier to incrementally expand capacity of RAID setups. ZFS promises RAID-Z expansion but it still hasn't been released. Btrfs has a flexible block allocator that supports incremental expansion as well as heterogeneous drive pools but unfortunately the parity support cannot be relied upon to this day.
