Web developers don't suck at making fast websites. Publishers demand tons of ads and tracking scripts. If the person signing the paychecks wants the page to have 15MB of sketchy third-party JS, then the page will have 15MB of sketchy third-party JS. AMP succeeded because Google brandished a stick that publishers cared about more: exclusion from the Top News carousel.
I'd also note that the whole reason this ad tech explosion even happened is a race to the bottom that Google itself facilitated. Google is selling us solutions to a problem that Google had a huge hand in causing.