Sketchfab clearly notes the original file format when you download a model, and lets you choose between it and various other converted formats. [Example: https://imgur.com/a/himJBfG ]
Blender doesn't fully support the USDZ format, causing the issues the author sees. [Source: https://docs.blender.org/manual/en/latest/files/import_expor... ] If the creator uploaded a USDZ file, downloaders would likely to run into similar issues.