Personal opinion but I think the old RAD approach in tools like Delphi where the components are laid out visually using the IDE but having the option of generating the components programmatically during runtime including attaching/detaching event handlers etc with state handled application-wide or within the context of the "form" is a much faster way to develop than to use the declarative approach in tools like Flutter where state management is really complex.
Also, one could develop custom components very easily in Delphi with custom draw events which would draw just the component.
I haven't used Delphi, but what you're describing sounds like Interface Builder, which I've used a lot. The biggest problem I had with IB was that it got quite tedious to specify all the constraints to make an app responsive to different screen/window sizes. Plus, you'd have some constraint in there and you wouldn't remember why it was there! IIRC you couldn't add comments. Also you couldn't diff the files because it was a ton of XML and merging them was really scary. I think bigger teams tended to stay away from IB/Storyboards altogether (storyboards were really bad because it was all centralized in one file).
Also, one could develop custom components very easily in Delphi with custom draw events which would draw just the component.