Here's why:
* Shi En's effect: Shi En's effect only negates the activation of a Spell/Trap Card that targets Shi En itself.
* Torrential Tribute's activation: Torrential Tribute doesn't target any specific monster. It activates by destroying all monsters on the field, and its effect doesn't directly target any monster.
* Response to Summon: While Torrential Tribute might be activated in response to Shi En's summon, it doesn't target him directly.
Therefore, Shi En's effect wouldn't apply, and Torrential Tribute would still activate and destroy all monsters on the field.