Every 3D model that is available to download or purchase on Sketchfab is also available in glTF, GLB, and USDZ versions that we generate automatically. The glTF/GLB versions are meant to be complete, robust exchange formats. However, the USDZ version is targeted mainly for viewing in AR QuickLook on iOS. As a result, the USDZ is optimized for file size and viewing in AR, but lacks more advanced features required by other applications that read USDZ files such as Apple's Preview and Motion. This is especially true of animated models. For example, when you try to open one of these models outside of Quicklook, you may see an empty scene:
A potential solution is to download a glTF or GLB version of the model from Sketchfab instead, and then use Apple's Reality Converter to export a new USDZ file.