A better example might be why we build stairs with a standard riser height and tread run. If you've ever accidentally tripped on an unusual or non-standard stair, you already know this.

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.