Here's why:
* Equip Spells target a specific monster: When you activate an Equip Spell, you target a specific monster on the field. It's not a continuous effect that stays attached to the monster regardless of changes.
* The effect of the Equip Spell applies to the target monster: The Equip Spell's effect (e.g., boosting attack, granting abilities) applies only to the specific monster it's attached to at the time of activation.
* Changes in control do not affect Equip Spells: If the equipped monster changes control (for example, being destroyed and sent to the graveyard, being banished, or being switched to another player's control), the Equip Spell will remain on the field and will not switch control.
Example:
You equip "Mystical Space Typhoon" to your opponent's "Blue-Eyes White Dragon." If your opponent somehow loses control of "Blue-Eyes White Dragon," the "Mystical Space Typhoon" will still remain on the field and be in your control.
Note: Some Equip Spells have specific effects that might change their control based on the equipped monster. However, this is not a general rule for all Equip Spells. Always read the card text carefully to understand its effects.