You bring up an interesting point - if AMP hosting is the same as a CDN, then why do companies use both solutions? "Because they appear the same" doesn't mean they are the same.
AMP requires that you consume other Google products, which requires that additional JS is loaded. When your mobile site doesn't use AMP, Google limits SEO rankings your mobile site can have. Google AMP requires your pages meet Google's Content Policies or they won't host them.
AMP and CDN delivered pages are architected differently and Google imposes restrictions and requirements that don't exist in a CDN.
AMP requires that you consume other Google products, which requires that additional JS is loaded. When your mobile site doesn't use AMP, Google limits SEO rankings your mobile site can have. Google AMP requires your pages meet Google's Content Policies or they won't host them.
AMP and CDN delivered pages are architected differently and Google imposes restrictions and requirements that don't exist in a CDN.