It's very achievable if you have full control over the site and you know what you're doing... However that's frequently not the case in many large companies.
Many times it just comes down to something along the lines of these pages being run by non-technical folks in sales/marketing that are just click-to-adding widgets/plugins/tags every week without ever removing anything. As a result, you see a lot of very low-hanging fruit make it into the final production site. It's not unheard of to see a website of a household name brand load in multiple versions of the entire jQuery library, for example. I've personally seen a major site from a recognizable brand that otherwise loaded in <1MB, but then proceeded to load Google Tag Manager and pull down an extra 15MB of JS/images.
My point is, I think you're discounting the "AMP enforces some good best practices / forbids some bad patterns" point.
Many times it just comes down to something along the lines of these pages being run by non-technical folks in sales/marketing that are just click-to-adding widgets/plugins/tags every week without ever removing anything. As a result, you see a lot of very low-hanging fruit make it into the final production site. It's not unheard of to see a website of a household name brand load in multiple versions of the entire jQuery library, for example. I've personally seen a major site from a recognizable brand that otherwise loaded in <1MB, but then proceeded to load Google Tag Manager and pull down an extra 15MB of JS/images.
My point is, I think you're discounting the "AMP enforces some good best practices / forbids some bad patterns" point.