The Literary Form was Sacred

A parable of Jesus is never used as a Story in itself, instead the parables are incorporated into the Stories. This was done for "security" to keep the literary form sacred. If they would have used a parable as a Story in itself, the literary form would be easy to recognize, and therefore easily duplicated, which would allow "anyone" to write in this same way.

By hiding the literary form, the integrity of the Scriptures were preserved.

Only certain people understood the literary form of the parable; it was not common knowledge that the Christian Scriptures were written in this way.