I recently figured out that I have some videos inside my Aperture library, the rest is referenced in a separate location.
Why can't Aperture make these videos available to iMovie for editing?
Because that is the way the program was written.
Seriously there is no way to answer that question. The designers of Aperture made this choice, whether it is because of some limitation in the underlying software or for other reasons no one can say.
It is as it is.
Retrieving data ...