Here's why:
* Raviel's Summoning Conditions: Raviel can only be Special Summoned from your hand by sending 3 monsters with different names from your hand to the graveyard.
* Graveyard Restrictions: There is no specific clause in Raviel's effect that allows it to be Special Summoned from the graveyard.
Important Note: There are some cards that can revive monsters from the graveyard, but these cards generally have specific requirements or restrictions. For example, "Call of the Haunted" can revive a monster from the graveyard, but the revived monster cannot attack the turn it is summoned.
Let me know if you have any other questions about Raviel or other Yu-Gi-Oh! cards!