As others are pointing out, AMP is often a poor substitute for the fully functional page. Like mobile sites used to be separate, barely functional versions of the desktop site, and Google penalized that behavior by deranking sites that had different content on mobile sites.

If I built my own version of an AMP page a few years ago, and served it without AMP, it would have been penalized. I don't know if that's still true, as I don't follow the carnival that is SEO anymore.