Among other things, pushing everything through the message bus allows for global message ordering, and security policies down to the individual message. Rewriting the internals would work in an embedded situation like that where every application is linking against the same version of libdbus, but that is not really the case on a desktop system, where there are multiple different D-Bus protocol implementations.

If applications have hard performance requirements, most D-Bus implementations do have support for sending peer-to-peer messages, but applications have to set up and manage the socket themselves.