It isn't only about the data. When people visit AMP pages from Google Search, they aren't visiting your site. They are visiting a restricted shell of it that is hosted on google.com with a left-right swipe that takes users off of your site instead of deeper inside it. It restricts your markup and monetization, and is one of the largest threats to the WWW at the moment. The web is supposed to be decentralized. Also, mobile-first doesn't mean mobile-everywhere.

What additional data does Google get through AMP that they don't get through Google Analytics?

Google Analytics is easily blocked while still having access to the content. Loading the full content from their AMP servers is an all-or-nothing scenario. It effectively moves Google from being a 3rd party to a 1st party and the content provider becomes the 3rd party instead

Nothing, OP is ignorant. Google does AMP to make webpages faster cause they make money from people clicking on links.

Google lock in, like Microsoft's MSN idea in 1995... Google wants you to never leave their servers, so they can track much more and much easier. Just bad for a free and open web.