AMP pages can be tested and proven to load fast. There's no way to guarantee that a non-AMP page will be fast for a user. The way AMP is designed allows Google to inspect the source and determine whether the page meets the requirements or not. Caching the page guarantees that the content Google tests is the content the user gets.

Imagine if Google rewarded plain old websites the same way they reward AMP sites. You'd have sites gaming the system by hiding ads and disabling web fonts just to get ranked higher, then taking a crap on "real" users visiting the site.