Here's why:
* Soul Exchange's effect: "Destroy 1 monster on the field. Then, you can Special Summon 1 monster from your Graveyard with the same Attribute as the destroyed monster."
* Requirement: The card specifically requires a monster to be on the field to be destroyed.
* No Target: It doesn't specify your side of the field, but if there are no monsters available to be destroyed, the effect cannot be activated.
Therefore, you need at least one monster on your field to activate Soul Exchange.