DC is an extension but that doesn’t explain its awkwardness. Extensions can be made to feel just as polished and integrated as native features and native features can be made to not fit in.
Take the native Solid Tools for instance. Their icons aren’t very clear, the color differs from the color of other native tools, the status bar text is nonsensical (“Select the first solid” - does that mean the solid to subtract from or subtract with?), they don’t honor components and the list just goes on.
I don’t know what happened but I think some really good design people left SketchUp quite early. The original features are among the best design I’ve ever seen! It has a top menu that you already know how to use, it has a context menu on right click (not panning or a programmable command), you can import images and other sketchup models simply by dragging them into the viewport.
At some point this changed and later features aren’t at all this well designed. Still we can’t install extensions by dragging an RBZ onto the viewport, while this in my view should have been implemented right away when RBZ support was first added.