The balance idea is raised and rejected, but as I understand it it's a huge part. The issue isn't about humans balancing on the thing (as with the examples of canoes or horses that are already stable at rest), but rather that a bicycle _does not balance unless it is moving_, and moreover making it balance depends on the fact that it has steering – fix the front wheel in place and you can't ride it.

This is not intuitive, except with the benefit of hindsight.