Because you need to convince business folks about stripping down bloat from your webpages. And doing that by citing sticks and carrots from Big-G is much easier than doing it on your own. So we can attribute this to: laziness of developers (who cannot make arguments to reduce bloat), boneheadedness of business folks (who cannot quickly understand why bloat is bad), tragedy of commons (my competitors are gaining valuable traffic by having their articles in special Google carousel at the top of search results, I am missing out so I must do AMP) etc.
There's no laziness or boneheadedness involved. Without the AMP carousel, making a fast website is a bad business decision. With the carousel, it's a good one.
The usability of your site just doesn't really matter if your business is to get traffic from Google and monetize it, which is what we are talking about here.
Because you need to convince business folks about stripping down bloat from your webpages. And doing that by citing sticks and carrots from Big-G is much easier than doing it on your own. So we can attribute this to: laziness of developers (who cannot make arguments to reduce bloat), boneheadedness of business folks (who cannot quickly understand why bloat is bad), tragedy of commons (my competitors are gaining valuable traffic by having their articles in special Google carousel at the top of search results, I am missing out so I must do AMP) etc.