3D models on Sketchfab are only available for free download or purchase if the owner allows it. You can search for free models to download here, and you can search for models to purchase in the Sketchfab Store here.
If a Sketchfab member decides not to offer their models as free downloads, that's their decision. You can always comment on the model page and request that the creator off it for download or purchase, but please be considerate of the artist and their work.