I think the main reason is they want money, if you pay them to store the files, then they get a monthly sub until you die. The accounting people love to see all this money and monthly subs come in.
I have a few family members use google/OneDrive, and the good thing is if your whole house burns/flood, your documents are recoverable.
These tools provide convivence to the end user.
Most people don't want to take the time to build their own servers or do everything manually with flash drives.