Too many cabinet types out there, and MAME is a project to preserve what it can. Believe me, if MAME didn't do artwork, a lot of it would have been lost by now.
The entire MAME concept is to make it preservable on pretty much any platform that might happen in the future. That means computers, tablets, even consoles if you're so inclined to port it (SDLMAME on PS3 Linux). Removing that functionality to modules does nothing more than to increase the complexity of the codebase for zero real gain.
You, as a user, would save probably 10k off the binary.
MAMEdev would gain about three extra bleeding ulcers maintaining it as a module when it's part of what they consider the core *meaning* of the project.
Try checking the MAME manual at http://docs.mamedev.org