Why? Because with each new generation of the organism, the new generation (as per dice throw/evolution analogy) would need the ability to randomly save the previous evolutionary advancement/s of the organism.
That is a rough idea of how evolution works. Very rough.
The offspring of the creature with the mutation would not revert back to the grandsire non-mutation.
That would be plain silly.
Facts don't lie or have an agenda. Facts are just facts