Tighter emmisions standards

I dont understand why then instead of adding things like air pumps ect that they wouldnt just increase the efficiency of the engine to burn cleaner, maybe this was not possible? Maybe like I have been told total timing is a number that really is built into the engineered charactersitics of the engine and again this couldnt be improved to burn any cleaner.

So instead of re-designing/re-configuring things ( assumig that was even possible/major re-vamps ) they instead decided to go the easier way and retard the timing (regardless of the innefiency ) and then just burn the un-used fuel in the exhaust before it reached the tailpipe?