Here's why:
* Timing: The card's text states "During your turn, you may sacrifice any number of creatures you control." This clearly indicates the ability can only be used during your own turn, not your opponent's.
* Activated Ability: Ultimate Offering is an activated ability, which means it is triggered by the player. Activated abilities can only be used on your own turn unless they specify otherwise.
Important Note: While Ultimate Offering can't be used on your opponent's turn, it can be used during your opponent's end step. This is because the end step is still considered part of your turn.