We moved away from it because frames are quite limited in what UI's they can express. I don't know that it's true that what we have now is much more complicated

We moved away from it because frames are quite limited in what UI's they can express. I don't know that it's true that what we have now is much more complicated, because it's pretty simple to build a multi-pane layout with CSS grid or flexbox. I think frames are an artifact of the time when people thought of HTML as a desktop publishing markup language. It has evolved into a generalized UI description language.