I don't see how a command line tool with 10 different flags is necessarily less "error prone" than UI. In fact, I have used many tools where a flag is confusing or has conflicts with another flag in an unexpected way, not to mention subtle issues like escaping.

I once had to pass along to the support team a command for one of our customers to run. It ultimately didn't work because someone along the chain A) autocorrected the spelling of the command name, B) converted the quotation marks to fancy “”, and C) converted the hyphen into some fancy dash.

It is easier to copy/paste. If the GUI has more than one step people might be confused. Also some GUIs are hard to see or read for people with vision handicaps.