I started a company to do exactly this a few years ago, and got to work with amazing companies testing their developer experience.

The problem is not the docs, it's Conway's law. One team designs the API, the other team designs the portal, and another team designs the SDK. The user has a holistic experience that cuts through each team.

That, and the docs are usually written first by the most technical person around, who has a hard time sharing the world view of a noob.