For services like these, whether it's Google One or Dropbox, R&D is the major expense. Not hard drive space. In order to recoup the R&D cost you have to segment customers by their willingness to pay. When you charge per GB you leave a lot of money on the table.
Google has spent billions in R&D for gmail, google docs/sheets, photos. And now I have to choose between deleting my old stuff or paying for extra storage. Figuring out where my data went is too much work, so I subscribed. Other people will delete old photos/attachments/files so they don't have to upgrade to the next pricing tier. Price segmentation works.