Two main reasons:
1. The obfuscation of AMP source URLs in Google search results is usually the biggest reason for the hate.
2. The "AMP Carousel" in Google search results is designed to keep people on Google and makes it much less likely people will leave to explore other pages on the publisher site (I can't find the article now but I recall reading a report from one publisher that showed a HUGE dropoff in users viewing an article and then going to view other articles on the site).
So it's less about the technology of AMP per se and more that, the way it's currently implemented, is about Google dictating more and more the way the web works.