[Tagging] Why isn't the amenity=parking object part of the relation ?

[OSMDoudou]

Do you mean you use plain polygons, and let data consumer derive relation observing the parking space polygon is situated within the geo boundaries of an amenity parking polygon ?

Or do you mean you use multi-polygon relations. [1]

[1] https://wiki.openstreetmap.org/wiki/Relation:multipolygon