Other people can answer it too.
There was an article published on
www.softimage.com back in the day crediting Charles Migos as the designer. I think he left Softimage before XSI hit the market, but the style has persisted. The look was originally based on the Softimage Digital Studio (Avid|DS) user interface, but modified to be less of a pillow button appearance to what you see today. XSI was originally supposed to be a component of Digital Studio, but when that effort proved futile, XSI was split off into it's own application and modified as needed, including refining the user interface.
Matt
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Matt Lind
Animator / Technical Director
Softimage certified Instructor: 3D, XSI