Because many managers hear "AMP gets boosted in search results and a special icon" and stop hearing anything after that. The number of times I've had to tell people "No, you can't do that with AMP" is crazy.
To get a ranking boost? For some search terms, moving to full AMP gives you a ranking boost catapulting you from page 13 or worse to the #1 result. So if you don’t use that, someone else will use that advantage.
Guardian for example runs their own AMP cache and gets in page 1 without Google AMP cache. Also AMP is one in many factors that is used to decide site relevance. Amazon and many other sites that don't use AMP gets featured in page 1 on mobile.