Being interested in Linux/Unix, file systems, crypto or whatever other technical topic doesn't necessarily mean you are also interested in developing graphical interfaces. That's an art form and discipline in itself. I suppose the people who make these tools are more interested in the tools' respective technical aspects than in interface or graphical design.

Moreover, many of these backup tools run on servers (or as an automated background process) where a graphical interface is more of a handicap than an asset.

I think that in open-source software "pretty interface" sounds often like "customer-oriented" which in turn sounds like "getting paid".