Because its machine parseable, it makes a lot of presentation options available that aren't available when you rely on a standard hyperlink without a data format with a standardized identification of the requested action.
> And he's right, it does make phishing easier.
Well, that depends on what the requirements are to have the client present the actions from the schemas: the current Google requirements, I would say, do not make phishing easier. You must register with Google for the schemas in the email you send to be recognized in Google products (e.g., Gmail) [1], and the registration is per-set-of-emails, and fairly specific as to the content, and appears to be manually reviewed [2].
[1] https://developers.google.com/gmail/schemas/registering-with... [2] https://docs.google.com/forms/d/1PA-vjjk3yJF7MLPOVKbIz3MBfhy...