It is far too expensive due to the power. Remember that you're not keeping one drive per unit of data going, but multiple drives for redundancy. Then there's the power used by the fans, power supply losses, and then the datacenter cooling costs...plus all the network infrastructure and so on. Some of those are pretty small but it adds up to a lot to keep, say, 6TB work of data online vs in a LTO cartridge sitting on a shelf.

Drive manufacturers, storage vendors, and cloud providers have been working on lowering opex/TCO for years. Bigger and bigger drives, helium which reduces aerodynamic drag, tiered storage where stuff is moved to slower and slower spindles the longer it goes without being accessed, for example...in some cases automatically getting pushed to tape. But you need systems to manage all that, systems that take power, have support contracts, and introduce a lot of complexity.

Eventually flash memory will get cheap enough - but flash degrades with time, too.