The way I understand it (consider this a rumor, but seems supported by evidence) is that many of the OOB components don't use the API, especially the older parts of the templating engine. Those parts can call directly into the body of the beast. For instance, I see no way to build a proper kudos leaderboard from the API but it exists as a widget.
Aurora appears to fix some of that but it's frustrating now.
I did catch a glimpse of some source code a long time ago at one of Lithium's conferences while talking to one of the lead devs at the time, complaining about this very issue. And yes it certainly wasn't FreeMarker 😉
