Here's why:
* Chaos Emperor Dragon - Envoy of the End has a specific summoning condition: "You can Special Summon this card (from your hand or Graveyard) by removing from play 1 of your LIGHT and 1 of your DARK monsters."
* Removing from play is a different effect from removing from the field. Removing from play means the cards are sent to the "Removed from Play" zone, which is no longer a valid location in the game.
* Special Summoning from the Graveyard is an effect that is not specifically granted by Chaos Emperor Dragon - Envoy of the End's summoning condition.
Therefore, even if you have Light and Dark monsters on the field, you cannot remove them from the field and use their removal to Special Summon Chaos Emperor Dragon - Envoy of the End from the Graveyard.
You would need to Special Summon it directly from the Graveyard by removing Light and Dark monsters from your hand or field.