Why would platform maintainers want to encourage the lowest common denominator apps that such an API would undoubtedly result in (as a standardized UI API by definition cannot leverage platform strengths)?

Apps like that get made anyway but as it stands at least there’s a healthy crop of smaller/indie native alternatives which often best the behemoths in UI/UX. That would likely disappear with the addition of a standardized UI API, as it would probably also come with the abandonment of the old specialized APIs.