Here's why:
* Gladiator Beast effects trigger only when they are Summoned. Their effects are activated by being Summoned to the field, not by simply being moved between zones.
* Returning cards to the Deck is a separate action. It doesn't count as Summoning, even if it's done by a Spell/Trap card.
For example:
* If you use "Return from the Different Dimension" to return a Gladiator Beast to your Deck, its effect will not activate because it was not Summoned.
* If you use "Call of the Haunted" to bring a Gladiator Beast from your graveyard to the field, its effect will activate because it was Summoned.
Important Note: There are some Gladiator Beast cards with specific effects that can activate under certain conditions, even if they aren't Summoned. However, the general rule for most Gladiator Beasts is that their effects only activate when they are Summoned to the field.