When SketchUp was first designed, it was regarded as a pure 3D modeling program. Limited text annotation such as dimensions was added because there was no other way to show such important information when presenting a model. But dimensions are only one small part of the annotation and documentation of a model for presentation.
Faced with increasing demand to be able to do serious documentation, the developers no doubt considered an integrated “paper space” facility similar to CAD. For whatever technical or other reasons, they chose instead to create a separate program to handle these 2D presentation aspects: LayOut.
That choice carried the implicit belief that all annotation work should be done in LayOut. So they stopped further development of equivalent capabilities in SketchUp. The Dimension Tool was retained, no doubt because it was already ingrained in people’s workflows and perhaps in various extensions, but they really didn’t want you to do serious work that way. For that reason, it has sat largely unchanged in SketchUp for a long time now. I doubt they will give it any priority now.
Ironically, as numerous posts in this forum complain, something in the way LayOut was built causes it to perform very poorly on large, complex presentations. The developers are working hard at improving LayOut. Having no insight into the internals, I don’t know whether they are fighting a losing battle against a fundamentally inefficient design or if there is some magic that will fix the issues.
