Graphical UI? There are dozens of text editors that work with TeX and most of them provide graphical UIs, with menus and buttons similar to a word processor. In fact, as soon you want to do anything evenly mildly 'advanced', word processors end up being visibly more complicated.
In a word processor, I can click the 'bold' button, or press Ctrl-b to switch into 'bold mode', or highlight some text, and use the button/keyboard shortcut to bold that text.
In a TeX editor, I can also click the 'bold' button or press a shortcut to auto-create a LaTeX bold environment `\textbf{}` with my cursor placed in-between the {}s. Alternatively, I can highlight text, e.g 'my text', and click the bold button or press the shortcut and the editor will wrap `\textbf{}' arount the text, producing '\textbf{my text]'.
Up to this point the two approaches are equivalent. But now say that I want to make all instances of 'important phrase' bold. With the TeX/editor approach, it's just like any other search and replace, I tell the editor to replace all instances of 'important phrase' with '\textbf{important phrase}'. In the word processor, I have to figure out how to click into an advanced search-and-replace and choose something about replace/add styles etc.
In LaTeX, for something complicated, I can figure it out and write my function(s) for it, which are easily re-usable. In a word processor, what one 'knows' in the case of doing something complicated is a series of mouse clicks through menus - which is not only more arcane than an explicit function, but is likely to be disrupted by version changes.