As I understand it, Galera doesn't work through the pluggable storage engine API, so adding it via the engine interface is not an option.  It intercepts the binary log capture process, implements a global transaction identifier and wire transaction state prototocol (wsrep), and is tightly integrated with InnoDB so as to trigger deadlocks on incoming replication write conflicts.  It is not possible to use Galera with TokuDB or MyISAM for this reason (though MyISAM is technically supported, it is not a good idea).  Because it only works with InnoDB and MyISAM (barely), it makes sense (at least to me) to make it a separate package.

Just my $.02