Here's why:
* Worm Yagan's Effect: Worm Yagan's effect states: "If this card is Normal Summoned: You can Special Summon 1 "Worm" monster from your hand or Graveyard."
* Special Summon Timing: The effect specifically states "If this card is Normal Summoned", meaning the effect only activates after Worm Yagan has been successfully Normal Summoned. It cannot be used to Special Summon itself during the same turn, as it is already on the field after its Normal Summon.
In summary, Worm Yagan's effect only allows you to Special Summon another "Worm" monster from your hand or Graveyard after it has been successfully Normal Summoned during that turn.