I expect that only someone from the core SketchUp development team could give you a solid answer. The rest of us can only speculate. And, as it involves proprietary technology, it isn’t likely that a SU team developer will give you a lot of technical details. I’m neither a SketchUp team member nor a game programmer, just someone with a fair amount of computer experience. So, to speculate away…
I think it involves the fact that SketchUp provides an editable model based on a geometry database in the main memory of the computer. The graphics processor (“GPU”) can do things such as orbit, zoom, pan, and paint polygons that have been passed to it. It can handle visibility and shading variations. It can also gather user inputs such as mouse clicks, drags. etc. But it must pass these inputs back to the computer’s central processor (“CPU”) for interpretation in terms of what they mean to the geometry database. After editing the geometry, the CPU must pass the resulting polygons back to the GPU for display. The possible changes to the content of the model are almost infinitely arbitrary, so there isn’t much the GPU can do to help. The new set of polygons could have no similarity to previous ones - but even if unchanged, it would be a complicated task for the CPU to tell the GPU which ones to keep vs replace. This division of responsibilities and round-trips limit performance.
In a game, I think that the contents of the model world are pre-determined to within scripted limits and the resulting possibilities are mostly pre-rendered as textured polygons that are uploaded to the GPU. There are also some kinds of shape morphing that the GPU can do with a set of polygons for animation. Most of each scene is background that, other than camera location, is static. Effects such as explosions consist of pre-rendered clips that are blended into the scene by the GPU. As a result, the CPU’s task is largely limited to ordering the GPU about what to do with the content that has already been passed to it.
These days a gaming GPU is actually quite a lot more powerful than a CPU, but not in ways that SketchUp can exploit.