Screens are the starting point of the mobile app lifecycle internally for the operating system (Android/iOS)  as well so it makes sense for App builder tools to start with it, Besides someone using a no-code tool would visualize their app via screens too.

But your distinction between Wix & WordPress as analogy for mobile apps is interesting, Could you be specific about what feature you found missing with current no-code tools for mobile development?

Btw, I think the success of WordPress is largely because of it being open-source and so hosting providers choosing it for deploying websites.