Why can I not add Site Users as recipients to Account Level Triggers?
As a single Site User, when you log in, you go straight to the Site Dashboard. If your scope is larger than one Site within an Account, you do get access to an Account Dashboard that lists the Sites you have access to. But you don't get access to Account Level settings and Trigger setup; that is reserved for the Account scoped users or wider scoped users. So for Account Level Triggers, they must be Account Users as they are the ones able to adjust/create the Triggers themselves and can be Acknowledged properly on the Account level.