You're leaving out the part about the software and operational support to handle failover, data integrity checks and correction, etc. You're not just tossing disks in a rack but managing a highly-available, highly-durable storage system on top of them.

You can definitely beat those rates at large enough scale but for a startup you're talking about dedicating a non-trivial chunk of your skilled technical staff's time in the hope of breaking even. That's an obvious move for Backblaze or Crashplan but for other companies it might be a dubious investment versus nailing the core product.