You're making an erroneous generalization. I don't feel up to providing details and photos right now, but let me know if you want/need the information.If a spin-on screws onto anything on the engine, and the filter is exposed, then it essentially "screws onto to engine" or "attaches - screws on to - the engine". Cartridge filters go "into a hole in the engine", and are not visible from the outside of the engine. Semantics.