DVCS is a relatively new technology. We're still figuring out the best ways of designing and using it.

To continue your filesystem analogy, filesystems were invented sometime around 1960, but it was over 20 years before everyone settled on Unix-style filesystems, where files are just a sequence of bytes. Eventually, they realized implementing version control or record structures at the filesystem level was unnecessary. DVCS implementations may similarly grow more alike and become simpler.