Because you're adding the
GeometryModel3D
element at the wrong place in your markup.
We can't tell you precisely where, because you forgot to include the parent element. According to the documentation, it looks like it needs to be inside a
ModelVisual3D/ModelVisual3D.Content
element:
How to: Create a 3D Scene - WPF .NET Framework | Microsoft Docs[
^]
<ModelVisual3D>
<ModelVisual3D.Content>
<GeometryModel3D>