This was a design choice as Z3D does generally not work that great for very strong separation values.
The way vorpX does Z3D you usually get less stereo nearby, but in the distance you often have stronger separation than you get with Geometry 3D (or in reality). In reality stereo vision ends about 10m in front of you while with Z3D one can freely adjust this point, so in the end the choice was made to amplify the effect where Z3D woks really well (in the distance) instead of introducing heavy artifacts.