Hi Eric,
I understand your frustration. I can't speak for Apple but I assume they don't offer this feature in GarageBand because it would complicate things for consumers. By limiting you only to software instruments, it becomes easy to "bounce" your software MIDI tracks down to audio files that can be stored in your iTunes playlist. Since the audio playback of your software instruments can be recorded within the software, the average consumer doesn't have to deal with cabling external devices, adding audio interfaces to record the audio output of the external keyboard, and routing MIDI data to specific channels within the instrument. GarageBand simplifies bouncing files under the "Share" menu (export to itunes, ringtone, song to disk, etc.).
In order to send the MIDI data out of GarageBand to your external MIDI instrument, you have to address how it gets to the instrument. Does your instrument have a built-in USB MIDI interface or does it require a third-party 5-pin din connector to USB MIDI converter interface? Older keyboards only have the traditional round 5-pin din MIDI connector ports. Newer keyboards include a built-in USB interface connection. The other big difference is that using an external device requires the user to learn how to navigate it's interface to assign sounds to various channels. I suppose Apple's focus upon ease of user interface is why they have never offered a MIDI out option. Keeping you within the GarageBand environment allows them to control the whole experience. Even if you are comfortable navigating your Disklavier's settings to assign sounds to channels, most consumers never bother to read manuals.
The bottom line is that, in order to simplify user interface, complicated options have to be removed. As a consumer-intended application, GarageBand mostly delivers this ease of use. I would recommend that you consider Logic. It has many additional features (like the ability to send MIDI to external devices) and the new version (Logic X) has many similarities in user interface with GarageBand.
Good luck