Let's suppose you had a fancy GUI wrapper around rsync which covered all the command line options, and a tiny output parser to display progress information. What more effort is needed to get a commercial, market-ready piece of software?
It's also not just a GUI. There is loads of code in there that does stuff. For example:
• Scheduling for scheduled backups
• A background daemon that looks for events to launch the UI for scheduled backups
• Manipulation of disks ownership stuff in order to allow backups with permissions to happen correctly
• Volume blessing, etc, in order to make bootable volumes
• Trash handling & maintenance for deleted files
• Deleting files (for removing old backups) And that was just off the top of my head. The app is many thousands of lines of code and it performs literally hundreds of functions. They're nearly all transparent to the user, and that is the idea.
As feelix says it is easy to burn 1000 hours on it. In addition to what feelix says you need to write help, build a site, do a lot of testing, write an installer etc. It does not compare at all to writing a script for technical users.