Because the Notifications do not allow any specified email address to be applied as the sender (only allows the admin address or the email field, neither of which is normally appropriate). Instead, one has to use the email actions, which do allow any address to be inserted. 

However the email action doesn't have the ability to generate the content template automatically as the notification does. As a result, every time I set up the form, I have to go through the notification process first, then copy the template out and then create an action using that content.

Suggest having a button to generate the email content in the actions as well.