> Question: If what makes AMP fast is the restrictions on size, JS, and CSS, and you know this and want to conform to this, why do you need to use AMP? Why not just develop your site like this anyways?

I think aside from the icon and the special treatment mentioned above, don't AMP pages get served directly from google and get preloaded on the search results page? So that's the other benefit, I guess. Whether a few less milliseconds from preloading on an already fast page really buys anything is another question.