While I agree that the commandline provides "a large number of utilities" that "can be linked easily", I don't think that's the whole story. While you could certainly design a circuit-diagram-style GUI for building commands (so-called "visual programming"), it would be a lot more tedious than typing, just because there's so much more bandwidth available on a 100+ button, two-handed input device than a two-or-three button one-handed input device. Also, a good deal of efficiency comes from terseness: I can imagine a GUI that would make it simple and visually obvious how the different atoms of a regular expression fit together, but such a GUI would spend a lot of visual bandwidth communicating which combinations are legal and which are absurd. Expressing a regular expression as a string gives you absolutely no such feedback, but if you already know the regex you want to use, it's an awful lot faster to type it.

Lastly, the command line gets a good deal of power from meta-programming: most commands deal with a loosely structured stream of text, and a set of commands is a loosely structured stream of text. Specifically in POSIX environments, primitives like command-substitution ("$()" or backticks) and the xargs command are powerful ways to write commands that construct and execute other commands. If your diagram-based UI contains a set of primitives for launching commands and working with text, you're going to have to add a whole new set of primitives for working with diagrams.