But imagine you've got an advertising department that wants three different ad networks, a couple different managers that want to see stats from a couple different analytics platforms, and and the designer wants to load a font from fontsquirrel and another one from typekit and another one from google web fonts, and as a developer who wants to keep the site fast you have to fight them every single time they want to add something else that slows your site down. Having the same fight every time, with everybody else saying "oh, it's just one request. and this one is really critical" it's hard to keep fighting that fight.
It's a lot easier to say "i can't do that, it doesn't work in AMP". If you can find a better way to convince large organizations that page load speed is a valuable metric, and more important that whatever other resource they want to load today, I'd love to hear it. But from what i've seen, AMP is the only thing that's had any success in tackling this problem.