why do we build with right angles, straight lines, regular curves, etc? Why not random angles, crooked lines, etc for style and "excitement"?
Why don't we assemble a furniture set from a random assortment of pieces from flea markets? People sense that that is ugly.
Users don't need to think about how to use them; they are ubiquitous and familiar, and therefore intuitive and automatic.
If every set of stairs (or, worse, if every stair in a set) was radically different, every time you approached some stairs you would have to think carefully about how to use them so you don't fall.
My point is that I don't find this to be case. Rather, consistent UIs, while functional, are also beautiful to me. The constituents of the UI can be designed with aesthetic taste, but the way it is all put together consistently and functionally has a beauty all its own.