The technical reason is that PTC, formerly the Parametric Technology Corp, doesn't default to creating parametric representations of imported geometry and so they don't really qualify as model tree items. The elements of the import are all accessible via the Import Doctor if any of them need to be deleted or modified as non-parametric items.
Catia patched parametric modeling onto their non-parametric modeler, so they kept the old functions to bridge over the old users.