Why it was decided that diplomatic offers must give a predetermined (non-random) result? What was wrong with the old system (except the spam offers from AI, which would be easy to fix without such dramatic changes)?
It was a gigantic pain in the ass, lent itself to neverending gameyness*, and real diplomacy doesn't work that way anyway.
*We all have saved and reloaded countless times until we got to annex that "Very Unlikely" vassal. Don't try to deny it.