You're paying for the hard drive, the server it goes into, the rack is in, the building it's in, and ask the power and maintenance for you to be able to access the data.
It's not even just the one drive. Data is stored across multiple drives, in multiple servers, hosted in multiple locations.
People pay the likes of Backblaze for it's reliability and them fool themselves into believing it's a dude with a HD stashed in a shelf somewhere.
Your data is scattered across multiple drives, but that doesn’t make it take up much more space. A terabyte stored in a RAID-5 array is 1.2 terabytes. Maybe the fixed price for unlimited storage part was a mistake, but the footprint overhead is relatively small. Also, keeping a year of history isn’t that much either unless you change every bit of the files multiple times. Using my BtrFS snapshots and Time Machine backups as a reference, normal data changes very little.
That's besides the point. The point is that storing data in a service such as Backblaze requires more infrastructure than just that one HD in that one server in that one location.