Thanks for the correction. I was going off the little bit of Linux code I've read, which seems to call most functions directly. And also another comment on this story. I don't know what to think now.
Linux is monolithic, but also modular. While drivers are almost entirely implemented with these kinds of objects, "core" modules have less pluggable functionality, and so you don't see it as much. For example, contrast the page cache code (that's basically mm/) with the VFS code (fs/). You'll notice how almost anything I/O uses these kinds of objects heavily.