Here's why:
* Zubaba Knight's effect is a continuous effect. It's always active and doesn't specifically target or interact with other monster effects.
* Zubaba Knight's effect only prevents the opponent from activating monster effects. It doesn't undo or reverse any effects that have already happened.
* Monster effects are usually activated during specific times. They are often triggered by being summoned, destroyed, or other events. Once a monster's effect has triggered, it can't be negated by Zubaba Knight's effect.
Example:
Let's say your opponent controls a monster with the following effect:
* "When this card is destroyed, destroy all monsters your opponent controls."
If Zubaba Knight is on the field, your opponent will still be able to activate this effect when their monster is destroyed. Zubaba Knight only prevents them from activating other monster effects, not from activating effects that were already triggered.
In short, Zubaba Knight can only prevent future effects from activating, it cannot undo past effects.