Because it is how many people want to work. It actually is more confusing individually grouping, or individually hiding, or individually locking objects.
We all know “layers” in SketchUp are not really layers. But this does not stop us from wanting a global collation / correlating / relation mechanism that looks similar to how the Layer Manager looks, but has more powerful abilities. Some have likened this to be something similar to Layer Groups, or tree structured layers, etc.
It would be very powerful if the “layers” concept went away, and it were replaced with a “relations” concept where an object could be tagged with more than one collating tag. (Notice I am purposefully avoiding the use of the term “layer”, as it makes little sense in 3D.)
So for example, A component could be tagged as belonging to the “Finishings”, the “Doors”, the “Fire Doors”, the “Second Floor”, and the “West Wing” variants, all at the same time.
Then depending upon what you wish to see, or print or edit, you can choose whatever combination of visibility switches you desire for the variant tags.